A baked-in content blocker like that of braves loses because it isn’t extensible.
In what way? I use(d) Firefox since the very first Firebird days, and Netscape Navigator before it, and I'm practically married to uBO (don't tell my wife!). That said, Brave's 'shields' blocker is just skinned uBO with some tweaks. It can add custom cosmetic filtering rules, additional adblock format filter lists, disable or enable JS (globally or per-site) and has built in fingerprint resistance. Aside from the differing UI, I genuinely can't think of anything overtly missing as such.