I’d be interested to hear what industries these sites are in as to the negative performance side of things.
I’m in the news publishing side and see positives from using AMP whereby it’s almost entirely adopted because site speed benefits and minimal features/functionalities required for an article in general.
Sure, if one of your competitive advantages is your creative or UX/UI functionality, AMP isn’t ideal for you. But if you’re just pumping out a bunch of daily content, with at most a video embedded, AMP works fine.