Sup2point0, I don't know if the old editor is entirely incompatible but Fandom's new design philosophy is to stick with the stuff the MediaWiki Foundation produces as much as possible. The MediaWiki Foundation stopped updating the old editor so it doesn't necessarily work with anything newer than MediaWiki 1.32. UCP is MediaWiki 1.33. For reference, Wikipedia is using the most-recent MediaWiki version which is 1.36.
The issue with Wiki Activity is the same as the old editor but worse. Wiki Activity was a truly custom feature by Fandom so there is no "almost compatible" version to even start from. Also, it is not just a bunch of styling changes. If that were the case, you could replicate it with no more than some CSS. Wiki Activity does differ from Recent Changes with regards to some of the content displayed as well as how much content is displayed.
I agree about the new notification system. Also, it currently has issues retrieving notifications beyond a certain count. That is going to be a huge issue for those of use that get a lot of notifications.
NOT Matipereira, there have been a lot of server issues recently. The issues you are describing may be results of the server issues. Also, if you don't mind me asking, what is your first account? It isn't obvious from your post.