Mostly ethical. If you care about privacy and not just a software junkie looking for the next shiny new thing, you should care about who is behind its development. Mozilla is not the same thing as another X company building Y browser.
Specifically though, they have a conflict interest. A good starting point to understand their position in the market:
Brave browser (Stage 1) is a Trojan Horse for their actual business which is a blockchain-based advertising platform (Stage 2).