Yeah I don’t seem to feel any performance or functionality degradation myself. What <X> ad blocking related feature has been made impossible with MV3?
I've seen some ads slip through the cracks nowadays, like I think Wikia ads for some TV shows I go to now have ads on them. They gave up on blocking Meta.
I believe the main things are that block lists have to be updated through the Chrome Web Store and there is an upper limit for things that can be blocked.
https://github.com/uBlockOrigin/uBOL-home/wiki/Frequently-as... has a few of the more practical concrete examples. Namely content can load without the blocker being ready to filter, CNAME cloaking can't be handled properly (in Chrom* at least), and the filter rules aren't as flexible.
Interestingly, the rule count limits themselves do not (currently) seem to be much an issue despite it being one of the more common concerns on the original MV3 announcement.