As far as I know Google doesn't let some pretty basic stuff from Crome into Chromium, for example translation (might even go as far as the inbuilt password manager). Potential forks either lose those features or have to implement them seperately.

Now that Manifest v3 is rolling out, apparently Google is able to somehow block the change from being easily reverted which is additional developmental load (or just show ads). Manifest v3 won't impact Brave too much since it only applies to extensions, while their adblocking is baked-in, but it's worse than uBO.

Firefox is fully open-source and doesn't artificially make enabling adblock an issue which might attract more simpler forks (as opposed to Opera, Brave and Edge having companies backing them, Firefox forks mostly have volunteer developers or open source collectives making them).