Just a simple example: You use a short loop sample and play it looped using the audio plugin. The gamer who's using Chrome will hear what you intended. The gamer using Firefox will hear a large gap between consecutive plays of the loop. Why? Because Chrome fully supports Web Audio, while Firefox doesn't.
I've not run into this problem myself, and I'd be interested in seeing an example. I bet 50p I can fix it :D
On a side note, when developing on Windows Phone, this issue even occurs with native development.
Another one: For every audio used in your game, you need 3 files. The original, a ogg and a aac version. Why? Because IE and Safari forces AAC and Firefox ogg. Chrome, thank god, doesn't care and just supports all.
But this doubles the amount of data needed for your game.
It doubles the filesize, but only the relevent audio is downloaded by the user. When packaging as native you can just remove the unnecessary audio files.
And a third one: Chrome supports WebGL, Firefox partly (at least not on my system), and IE will never ever support it, as long as DirectX exists.
IE does now support WebGL and I've not had issue with Firefox myself (FirefoxOS on the other hand...).
So yeah, I'd contest your points :)