XDG User dirs are cool, i agree. But that’s not really the problem here

They’re very reliable from my experience

Just checked, you’re right about Google. Microsoft does allow you to use any app though. It’s funny that the “EEE” Microsoft is less anti-user than the “Don’t be evil” Google. But anyway, seeing how Google locks it down, I’m sure there must be others too. So you’re right

Which services aren’t supported?

It does have automatic Android cloud backups and does support local backups, which also supports backing up to Nextcloud.

For anyone looking at this in the future: https://www.zoho.com/mail/help/adminconsole/catch-all-setup.html

No it isn’t. Works as intended for me on both Firefox and Brave.