mobi (KF7) is really limited
No CSS, it's HTML 3 with a few inline styles and some mobi extensions.
Very limited font and language support (no embedded fonts).
It's never better than azw3.
Already obsolete before Amazon bought Mobipocket in 2005.
Only the K1, K2, DX and DXG must use mobi. All other Kindles can use azw3 (KF8).
The azw (without 3) is mobi with later Amazon encryption.
Can't easily be edited.
AZW3 / KF8
Basically Amazon version of epub.
Uses CSS and similar HTML to HTML5.
Supports extra internal and external fonts
Supports multiple languages / wrting systems
Supported by all Kindles except the K1, K2, DX and DXG, though some earlier Kindles need updated to get AZW3. It was backported, where possible because it's so much better.
AZW3 is easily edited in Calibre.
https://www.amazon.com/gp/help/custo...MQC26VQQMM8XSW
No plain novel is better in mobi than azw3. There are only a few edge cases of mobi, not actually reflowable novels, that don't work as AZW3 (I've never actually encountered them).
Mobi is so bad it should never have been used by Amazon in 2007 for 1st Kindle. The precursor to epub2 came out in 1998 and was better and open. Amazon wanted the Mobipocket encryption instead of Adobe (which they were using). They did ebooks befor Kindle launched in 2007 and bought Mobipocket (world's biggest ebook seller) in 2005. Mobipocket's mobi encryption was cracked, so Amazon moved to azw (mobi with amazon encryption before they released azw3 (KF8).
The ONLY reason ever to format ebooks in mobi is if you only have a K1, K2, DX (batteries likely dead) or the DXG (better screen and maybe dead battery).
The K1 is terrrible. The K2 & DX have poor screens.