Google is surely terrified of this “challenger” being built on a foundation (chromium) they have complete control over.
Brave and all the other browsers based on Chromium have said that they can only keep support for Manifest V2 extensions as long as support remains in Chromium; meaning they don't have the ability to maintain it themselves and are at the mercy of upstream.
P.S. Brave has integrated ad-blocking (i.e., blocking not implemented as an extension) which some say is comparable to Ublock Origin.