The compat story for Brave and Firefox is about the same. Tracking protections in Brave broke sites more often than Firefox when I used it for a few months, and there was a recent incident where iCloud was being blocked in Brave.

And despite what a lot of people say, engine interop is only getting better thanks to initiatives like Compat2021. More sites work well under Gecko than ever before. Anyone remember IE6 era where Firefox needed bug-for-bug compatibility? It's been worse folks.