You're downplaying the implication of "moving one part of the architecture to the client". This inversion of the model means the user is sovereign over their data. It means ad fraud (responsible for billions lost) is dealt a massive blow. It means the publisher doesn't have to watch hand after hand take from their revenue before it reaches them. It means the user, finally, can get paid for the limited commodity called "Attention".
As for what guarantees Brave won't do evil, look at our design principles. When it came to syncing data from one device to another, we designed it in such a way that we could never see what data was moving from device to device. Encryption on the machine shields all eyes, including our own, from your data. Consider also Payments--the user's ability to support content creators. Even here we implemented the ANONIZE protocol to protect your anonymity and privacy. That alone wasn't satisfactory, so we also channel the info through a VPN to mask your location as well.
All that we do remains open, and engineered in such a way that Brave couldn't abuse your trust even if we wanted to. This future friendly principle ensures that Brave couldn't turn sour on you in the future; not without a massive re-engineering effort beforehand. But even then, all of our code is open and auditable.
Google was "Don't be evil," but Brave is "Can't be evil." The difference is subtle, yet profound.