I’m not familiar with any models not loading mobi. The only caveat is that there is a sub-format of mobi that’s fixed rather than reflowable (like, a PDF vs. a TXT), and that’s never been supported. But I’ve only come across those once in a blue moon in the wild.
ePubs seem more popular than mobi though, so I think that’s where most of the “converting for kindle” happens.
.mobi predates Kindle, but it was the first format on the original Kindles. Amazon bought the company which made it. Amazon no longer uses the original mobi format, but keeps it for backwards compatibility. Most non-Amazon sources for Kindle books still use mobi.
