People don't like the creator of Brave because he's supposedly anti-trans. He donated to some anti-trans political group iirc.
The browser also has some crypto stuff (web advertisment replacement, block chain based decentralized browser sync), and a lot of people hate crypto these days.
Personally I think it's a good browser, the web needs advertising revenue to function and it's solution to replacing web ads with optional browser ads that still pay the websites you visit seems like a decent solution. I respect the push to use a non-chromium browser, but personally I rely too much on browser tab groups to use anything Firefox based.
I've tried both of those, tree style tabs kinda works, but isn't ideal. It's also not an option on mobile at all, and I prefer to use the same browser for mobile and desktop for tab sync/etc.
I used Firefox on desktop and mobile for a few months this past year, but never got as nice of a work flow going as I had with Brave. Then a Firefox update for mobile broke the browser for a week or two (crashed on launch, resetting app data/reinstalling didn't help) and I went back to Brave, and realized how much I missed tab grouping and some other stuff.
I'm keeping Firefox installed, and I'd be happy to switch back someday if tab grouping gets ported over.
To anyone wondering about the whole "homophobe thing", here is a (hopefully neutral? If you have different sources please share them as well!) wiki link to the drama.