For me it's not a matter of trust, it's just that there's no customizability to the ad-blocking on Focus, even at the level of choosing blocklists to subscribe to, or adding particular elements to be blocked.

I don't want a company that releases a "privacy focused browser" with telemetry enabled by default deciding which tracking is "ok" and which isn't. I love Firefox compared to other browsers but there's definitely some user-hostile choices being made at Mozilla and I'd rather they just not get involved in ad blocking (other than ensuring the add-on architecture allows proper blockers to do what they need to get the job done right).