Yeah - the tweaks can be substantial and they have the flexibility to do more. Brave has a whole development pipeline for incoming Chromium changes so they can intentionally bring in code they want and avoid bringing in code they don’t want, like FLoC, or changes that would conflict with their own tweaks. But I don’t think many other browsers change a ton in the engine itself, so you effectively end up with them getting as much customization of Chrome, Firefox, or maybe of Safari (mostly of Chrome) as Apple allows browsers on its platform.

Having older versions isn’t generally an advantage, unless you’re trying to test for compatibility or something similar. It means you’re more vulnerable to known threats that have been patched in current versions of browsers