I use Kindle Previewer for all my conversions and never seem to have the issues I hear about with calibre. I highly recommend it. I like that it gives you a compile log so you can see if anything didn't go as planned. Sometimes a file path is wrong within the HTML of the book, and that helps you trace it down quickly and resolve the issue. Debugging someone else's HTML and ePubs can be a PITA, but this makes it easier.