I've tested the new Edge ePub addon with the semi-official
Reflowable Content Tests epub3 test file and it passed most of them except for VP8 and MathML support. (Unlike most other epub3-enabled apps it doesn't support JavaScript.)
A nice touch is also the built-in TTS support. BTW, it apparently also supports popup footnotes if they were encoded using the preferred iBooks footnote syntax.
I.e., the new Edge browser epub add-on should be suitable for most fiction books, but not for MathML-heavy textbooks or books with media overlays and other advanced epub3 features.
(It does support embedded audio/video files, though.)