Out of curiosity, which sites are these? Do they also similarly break with Firefox or other non-Chrome browsers or only MS Edge?

Non-Chromium derivatives are blocked as well, yes, but neo-Edge is a Chromium derivative.

Google Stadia was one example:

https://www.trustedreviews.com/news/why-stadia-access-blocke...

Which websites?

Same with Firefox.

They shouldn't be blocking Firefox either, but let's be clear-- Chromium Edge makes even less sense given it's a "cousin" of Google Chrome.

I wonder why Microsoft isn't doing that, they could just use Chrome user agent for all Google websites.