Muon was a fork of electron, which included many patches for features required for a variety of desktop apps. The amount of packages ensures that it takes time to upgrade to new chromium releases (regression testing with the patches).
Since brave is just a browser, it does not need all of those extra patches and features. So it is simpler and easier to just roll with chromium directly.
Nah. Electron does not prioritize chromium version updates over everything else. They generally do so at their own pace, instead focusing more on other features and bug fixing.
It works fine for their intended userbase. For a browser that might be an issue due to exploits and vulnerabilities.
Since brave is just a browser, it does not need all of those extra patches and features. So it is simpler and easier to just roll with chromium directly.