There are many downsides when you don't control the standard. Microsoft is not in the meetings when Google PMs decide the roadmap nor will they have any clout in directing its future or adding its own features. It's merely a consumer.

The main thing about AMP is that it gives consent to the search engines which is why Microsoft is happy to use it. The consent allows search engines to display content without directing users to the publisher platform. And therein lies the rub.