I guess I'm just lucky. Calibre has never screwed up any of my epubs, although I keep my original just in case. Of course I don't do anything more complicated than a table
Calibre assigns every, single html element a class. That's the part I can't tolerate (but I understand why they've chosen to do it that way). I want bare-naked tags, except for the handful of paragraphs/spans/etc... that need
special styling. But I fully admit I'm just extra finicky when it comes to that sort of coding convention. I'm a rabid minimalist.