Well then, as someone with much more expertise in audio than video, Opus not being compatible with MP4 is a big win for WebM in my book.
...unless you do streaming in the same manner that YouTube does whereby you just deliver completely independent streams for audio and video, therefore allowing you to use MP4 and WebM streams concurrently (sometimes a recently-uploaded video will have VP9 video encodes but only AAC audio encodes for whatever reason;
here's an example video of such that has VP9 video yet only AAC audio as of this comment).