The big problem with FLoC, as I see it, is that it makes fingerprinting
vastly easier. Your FLoC bucket narrows you down to one of several thousand users, rather than one of several million, and that's before fingerprinting applies.
Ironically, it seems that FLoC makes user tracking easier, not harder.
I see no upside in FLoC for me as a user, and plenty of potential downside. I'm glad I use Firefox.