I tried converting Ulysses and got the following error
calibre, version 3.15.0
ERROR: KFX conversion failed: <p><b>Cannot convert Ulysses</b><br><br><b>Conversion error:</b> Kindle Previewer error: Error(YJConvValidator):E120042: Tables with number of columns more than 8 and rows lesser than 65 are currently not supported.</p>