What's the alternative if you want full ad-blocking in a Chromium browser? I use Firefox normally and wouldn't trust Brave, but there are some sites FF doesn't work with, so it's understandable why some people wouldn't use it.

Helium(comes with UBO) or Ungoogled Chromium with UBO sideloaded.

What doesn't work with Firefox?

I don't remember, it's usually a random poorly-made site like the DMV or a budget airline. Reddit used to have issues but not anymore.

There's a minimal version called Brave Origin https://brave.com/sv/ublock-origin-alternative/