I never really compared the source to the converted mobi, but overall I can't really complain.
Usually if the formatting is a little screwed up I assume it's the source's fault. :)
Almost every book I read nowadays is an epub converted to mobi.
Honestly, the bigger issue I have is the inability to put the sideloaded books into a series "folder".
Instead I had to set Calibre to add " " string to the book tile in the metadata. :/
I guess OP wasn't aware. Or it just wouldn't fit the meme. ¯\(ツ)/¯