I just converted the following two ePubs to Mobi and they both came out fine.
The Secret Adversary
https://www.mobileread.com/forums/showthread.php?t=53913
The Mysterious Affair at Styles
https://www.mobileread.com/forums/showthread.php?t=54721
The only thing that didn't convert well was the SVG cover used in
The Mysterious Affair at Styles.