I basically agree with PeterT. In many cases epubs would do as well as kepub.epubs or even better. But there are some points that kepub.epubs are better at, to mention those that come immediately to my mind:
- zoom of images
- better support of vertical writing
- adjustable line spacing in Japanese language books (In Japanese language books line spacing set in the css file is not honored in epubs. This is extremly tiresome with horizontal writing in combination with furigana, since the lines overlap slightly. Note: one can not change the line-spacing, margins, justification via the menu irrespective whether it is an epub or a kepub.epub)
- pop up preview of footnotes
- automatic font substitution for those characters that are not supported by the selected font (does not always look nice, but it is still better than question marks, boxes, crossed boxes or nothing)
- support of TOC in epub 3 without epub 2 fallback toc.ntx