Whether or not a particular muzzle device negatively or positively affects accuracy depends on a lot of variables, harmonics, barrel profile, barrel stress for a given heat, whether it's hammer forged, or button rifled, cryogenically stress relieved, how hard it's torqued down, how hard the barrel is torqued into the receiver upper, the crush or peel washer used for timing it, as mentioned above. Then there's ammo weight, the speed and pressure curve characteristics of the particular load and powder, the differences between NATO spec and SAMMI .223 brass, primer strength... goes on and on...
I'm sure having or not having one matters if you're going for absolute bench-rest accuracy, or competition, but the individual variables get to be so diverse, it's one of those black-arts kind of things where there's no broad conclusions to be drawn.