had at least few Conversion Errors maybe like 1 so far from KEPUB to the KFX Format, also prefer it over the RB Format since that one made all ebooks black and white without color images, so now if
Basically KEPUB input to KFX output works great;with, only few Conversion Errors so far 
correction in which I had one ebook that got an error from KEPUB input to KFX output, but still prefer it over RB format, I just have to work with them in different inputs to get the few that still get Conversion Errors fixed in any case.
also when using kepub as the input if you don't unselect strip kobo spans it removes the images with that option turned on I noiced that it loses the images in the ebook if it selects strip kobo spans
one fix for some Conversion Errors would be to use styling tab, with filter style information to select what style information you want completely removed like say if the problem is in floats or padding but I am guessing disabling from of the CSS may fix them from getting conversion errors on converting but I am guessing about that anyway it may or may not help. Below information from the ebook that failed to convert from KEPUB to AZW8 ext/ format kfx, so I just have to play with the input format until I found one that works, disabling the CSS in padding and floats and fonts didn't help with this ebook in any case.
Here text below from log of failed ebook if that helps again remember I replaced all kfx with azw8 and it works for most of my ebooks, but a few.
Spoiler:
Converting "C:\Users\ROBERT~1\AppData\Local\Temp\calibre_ddpz 6l\_ixbuh.epub" to KDF/KPF
Preparing EPUB for conversion
Adjusted NCX TOC reference to body element id (UGI0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0001_split_000.html
Adjusted NCX TOC reference to body element id (4OIQ0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0005.html
Adjusted NCX TOC reference to body element id (8IL20-7a3a6d49f4ab4a0abf8a08274945da27): text/part0009_split_000.html
Adjusted NCX TOC reference to body element id (AFM60-7a3a6d49f4ab4a0abf8a08274945da27): text/part0011_split_000.html
Adjusted NCX TOC reference to body element id (DB7S0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0014_split_000.html
Adjusted NCX TOC reference to body element id (F8900-7a3a6d49f4ab4a0abf8a08274945da27): text/part0016.html
Adjusted NCX TOC reference to body element id (I3QM0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0019_split_000.html
Adjusted NCX TOC reference to body element id (K0RQ0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0021_split_000.html
Adjusted NCX TOC reference to body element id (LTSU0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0023_split_000.html
Adjusted NCX TOC reference to body element id (NQU20-7a3a6d49f4ab4a0abf8a08274945da27): text/part0025_split_000.html
Adjusted NCX TOC reference to body element id (RL0A0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0029.html
Adjusted NCX TOC reference to body element id (SJGS0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0030_split_000.html
Adjusted NCX TOC reference to body element id (10DJ40-7a3a6d49f4ab4a0abf8a08274945da27): text/part0034_split_000.html
Adjusted NCX TOC reference to body element id (1394Q0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0037_split_000.html
Adjusted NCX TOC reference to body element id (173720-7a3a6d49f4ab4a0abf8a08274945da27): text/part0041_split_000.html
Adjusted NCX TOC reference to body element id (1AT9A0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0045_split_000.html
Adjusted NCX TOC reference to body element id (1ENBI0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0049_split_000.html
Adjusted NCX TOC reference to body element id (1IHDQ0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0053_split_000.html
Adjusted NCX TOC reference to body element id (1MBG20-7a3a6d49f4ab4a0abf8a08274945da27): text/part0057_split_000.html
Adjusted NCX TOC reference to body element id (1Q5IA0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0061_split_000.html
Adjusted NCX TOC reference to body element id (1UU540-7a3a6d49f4ab4a0abf8a08274945da27): text/part0066_split_000.html
Adjusted NCX TOC reference to body element id (20R680-7a3a6d49f4ab4a0abf8a08274945da27): text/part0068_split_000.html
Adjusted NCX TOC reference to body element id (24L8G0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0072_split_000.html
Adjusted NCX TOC reference to body element id (25JP20-7a3a6d49f4ab4a0abf8a08274945da27): text/part0073.html
Adjusted NCX TOC reference to body element id (26I9K0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0074_split_000.html
Adjusted NCX TOC reference to body element id (2ACBS0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0078_split_000.html
Adjusted NCX TOC reference to body element id (2E6E40-7a3a6d49f4ab4a0abf8a08274945da27): text/part0082_split_000.html
Adjusted NCX TOC reference to body element id (2I0GC0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0086_split_000.html
Adjusted NCX TOC reference to body element id (2MP360-7a3a6d49f4ab4a0abf8a08274945da27): text/part0091_split_000.html
Adjusted NCX TOC reference to body element id (2OM4A0-7a3a6d49f4ab4a0abf8a08274945da27): text/part0093_split_000.html
Adjusted NCX TOC reference to body element id (2RHM00-7a3a6d49f4ab4a0abf8a08274945da27): text/part0096.html
Launching Kindle Previewer EpubToKFXConverter (version 3.5.0)
Conversion to KDF failed. Retrying with log capture enabled
Launching Kindle Previewer EpubToKFXConverter (version 3.5.0)
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
******************** EpubToKFXConverter_log.txt ********************
log4j:WARN No appenders could be found for logger (a.b.a.e).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See [url]http://logging.apache.org/log4j/1.2/faq.html#noconfig[/url] for more info.
Oct 04, 2016 3:57:43 PM java.util.logging.LogManager$RootLogger log
INFO: RESPONSE1: MASTERMOBI CREATION SUCCESSFUL
Oct 04, 2016 3:59:35 PM java.util.logging.LogManager$RootLogger log
INFO: Error in conversion.
******************** log_conversion.txt ********************
Info(YJEpubAdapter):I00102: Converting File: C:\Users\Roberts Pc\AppData\Local\Temp\calibre_ddpz6l\wectlp\yxgvw4 h0to74qiht6gbb\b5q74ch70lnus8y6dd7r\conv_tmp\_ixbu h.epub; Processing html file C:\Users\ROBERT~1\AppData\Local\Temp\calibre_ddpz6 l\wectlp\yxgvw4h0to74qiht6gbb\b5q74ch70lnus8y6dd7r \conv_tmp\temp\extractedEpub\text\part0027_split_0 00.html
Info(YJEpubAdapter):I00102: Converting File: C:\Users\Roberts Pc\AppData\Local\Temp\calibre_ddpz6l\wectlp\yxgvw4 h0to74qiht6gbb\b5q74ch70lnus8y6dd7r\conv_tmp\_ixbu h.epub; Encountered an unknown tag or style {body}{body-margin-right-importance=nullspecial_unit}{}
Info(YJEpubAdapter):I00102: Converting File: C:\Users\Roberts Pc\AppData\Local\Temp\calibre_ddpz6l\wectlp\yxgvw4 h0to74qiht6gbb\b5q74ch70lnus8y6dd7r\conv_tmp\_ixbu h.epub; Encountered an unknown tag or style {body}{body-margin-left-importance=nullspecial_unit}{}
Info(YJEpubAdapter):I00102: Converting File: C:\Users\Roberts Pc\AppData\Local\Temp\calibre_ddpz6l\wectlp\yxgvw4 h0to74qiht6gbb\b5q74ch70lnus8y6dd7r\conv_tmp\_ixbu h.epub; Processing html file C:\Users\ROBERT~1\AppData\Local\Temp\calibre_ddpz6 l\wectlp\yxgvw4h0to74qiht6gbb\b5q74ch70lnus8y6dd7r \conv_tmp\temp\extractedEpub\text\part0027_split_0 01.html
Info(YJEpubAdapter):I00102: Converting File: C:\Users\Roberts Pc\AppData\Local\Temp\calibre_ddpz6l\wectlp\yxgvw4 h0to74qiht6gbb\b5q74ch70lnus8y6dd7r\conv_tmp\_ixbu h.epub; Encountered an unknown tag or style {body}{body-margin-right-importance=nullspecial_unit}{}
Info(YJEpubAdapter):I00102: Converting File: C:\Users\Roberts Pc\AppData\Local\Temp\calibre_ddpz6l\wectlp\yxgvw4 h0to74qiht6gbb\b5q74ch70lnus8y6dd7r\conv_tmp\_ixbu h.epub; Encountered an unknown tag or style {body}{body-margin-left-importance=nullspecial_unit}{}
Info(YJEpubAdapter):I00102: Converting File: C:\Users\Roberts Pc\AppData\Local\Temp\calibre_ddpz6l\wectlp\yxgvw4 h0to74qiht6gbb\b5q74ch70lnus8y6dd7r\conv_tmp\_ixbu h.epub; Processing html file C:\Users\ROBERT~1\AppData\Local\Temp\calibre_ddpz6 l\wectlp\yxgvw4h0to74qiht6gbb\b5q74ch70lnus8y6dd7r \conv_tmp\temp\extractedEpub\text\part0028.html
Info(YJEpubAdapter):I00102: Converting File: C:\Users\Roberts Pc\AppData\Local\Temp\calibre_ddpz6l\wectlp\yxgvw4 h0to74qiht6gbb\b5q74ch70lnus8y6dd7r\conv_tmp\_ixbu h.epub; Encountered an unknown tag or style {body}{body-margin-right-importance=nullspecial_unit}{}
Info(YJEpubAdapter):I00102: Converting File: C:\Users\Roberts Pc\AppData\Local\Temp\calibre_ddpz6l\wectlp\yxgvw4 h0to74qiht6gbb\b5q74ch70lnus8y6dd7r\conv_tmp\_ixbu h.epub; Encountered an unknown tag or style {body}{body-margin-left-importance=nullspecial_unit}{}
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at java.util.jar.Manifest$FastInputStream.<init>(Mani fest.java:332)
at java.util.jar.Manifest$FastInputStream.<init>(Mani fest.java:327)
at java.util.jar.Manifest.read(Manifest.java:195)
at java.util.jar.Manifest.<init>(Manifest.java:69)
at java.util.jar.JarFile.getManifestFromReference(Jar File.java:199)
at java.util.jar.JarFile.getManifest(JarFile.java:180 )
at sun.misc.URLClassPath$JarLoader$2.getManifest(URLC lassPath.java:944)
at java.net.URLClassLoader.defineClass(URLClassLoader .java:450)
at java.net.URLClassLoader.access$100(URLClassLoader. java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java: 368)
at java.net.URLClassLoader$1.run(URLClassLoader.java: 362)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.j ava:361)
at java.lang.ClassLoader.loadClass(ClassLoader.java:4 24)
at sun.misc.Launcher$AppClassLoader.loadClass(Launche r.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:3 57)
at com.amazon.adapter.common.e.a.a(Unknown Source)
at com.amazon.adapter.common.e.a.j(Unknown Source)
at com.amazon.adapter.common.app.EpubAdapterApp.main( Unknown Source)
******************** log_cap.txt ********************
VALUE0pt
cssValueType 1
cssValueType CSS_PRIMITIVE1
VALUE0pt
cssValueType 1
cssValueType CSS_PRIMITIVE1
VALUE0pt
cssValueType 1
cssValueType CSS_PRIMITIVE1
VALUE0pt
cssValueType 1
cssValueType CSS_PRIMITIVE1
VALUE0pt
cssValueType 1
cssValueType CSS_PRIMITIVE1
VALUE0pt
cssValueType 1
cssValueType CSS_PRIMITIVE1
VALUE0pt
cssValueType 1
cssValueType CSS_PRIMITIVE1
VALUEcenter
load finished
Tag, Style extraction complete!
Oct 04, 2016 3:57:06 PM com.amazon.D.c a
INFO: Done reading canonical combination in time 22294
Oct 04, 2016 3:57:07 PM com.amazon.source.analyser.application.SourceAnaly serApp main
INFO: C:\Users\Roberts Pc\AppData\Local\Temp\calibre_ddpz6l\wectlp\yxgvw4 h0to74qiht6gbb\b5q74ch70lnus8y6dd7r\conv_tmp\_ixbu h.epub,true
******************** log_createMM.txt ********************
Info(prcgen):I1040: The document identifier is: "file title name"
Info(prcgen):I1041: The file format version is V6
Info(prcgen):I1031: Saving PRC file
Info(prcgen):I1032: PRC built successfully
Info(prcgen):I1016: Building enhanced PRC file
Info(prcgen):I1007: Resolving mediaidlinks
Info(prcgen):I1011: Writing mediaidlinks
Info(prcgen):I1009: Resolving guide items
Info(prcgen):I1017: Building PRC file, record count: 0007921
Info(prcgen):I1039: Final stats - text compressed to (in % of original size): 43.51%
Info(prcgen):I1041: The file format version is V8
Info(prcgen):I15000: Approximate Standard Mobi Deliverable file size : 0009803KB
Info(prcgen):I15001: Approximate KF8 Deliverable file size : 0014098KB
Info(prcgen):I1036: Mobi file built successfully
******************** Failure Reason ********************
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
Python function terminated unexpectedly
{"title": "azw8 conversion failed", "det_msg": "", "msg": "title<br><br><b>Kindle Previewer error:</b> Exception in thread \"main\" java.lang.OutOfMemoryError: Java heap space", "level": "error"} (Error Code: 1)
Traceback (most recent call last):
File "site.py", line 132, in main
File "site.py", line 109, in run_entry_point
File "site-packages\calibre\utils\ipc\worker.py", line 190, in main
File "site-packages\calibre\gui2\convert\gui_conversion.py", line 31, in gui_convert_override
File "site-packages\calibre\gui2\convert\gui_conversion.py", line 25, in gui_convert
File "site-packages\calibre\ebooks\conversion\plumber.py", line 1222, in run
File "calibre_plugins.azw8_output.__init__", line 69, in convert
File "calibre_plugins.azw8_output.__init__", line 124, in convert_from_epub
File "calibre_plugins.azw8_output.__init__", line 162, in report_failure
calibre.ebooks.conversion.ConversionUserFeedBack: {"title": "azw8 conversion failed", "det_msg": "", "msg": "title<br><br><b>Kindle Previewer error:</b> Exception in thread \"main\" java.lang.OutOfMemoryError: Java heap space", "level": "error"}