Do brakes affect accuracy?

Accuracy? Not inherently. A properly installed and well produced brake should leave your rifle with all the accuracy potential it had before.

That said – your zero will almost surely change and a load or ammo that had previously been accurate or inaccurate in your rifle may now be less or more accurate. The reason is that you’re hanging weight on the end of your barrel, and that will effect harmonics. For reloaders, this is often fixed by adding or dropping a couple tenths of charge weight. For guys shooting factory ammo, this may mean trying some new brands, varieties, or even just lots of ammo. For many customers there is no change at all, just know that it’s possible and be prepared to deal with it.

Suppressors will have a much more profound effect, it’s just a larger application of the same principal.