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could've been worse
I'd like to point out that Cofagrigus was solidly UU in gen 5 and definetely usable in OU.You do realize if you bring pokemon from NU (like cofag) only because Of 1 pokemon (Rocky helmet is almost always terrible.)
However I agree with the point that making Rocky helmet the go-to item on Skarm, Ferrothorn and Garchomp isn't optimal. And while there is a fair share of things that can survive against Mega Kanga for a few turns, most of them have difficulties damaging it directly, instead having to resort to passive damage through Will-o-wisp and Rocky helmet. WoW only really works if there's no Heal bell suuport, and Rocky helmet is bypassed by non-contact moves and probably won't kill Kanga before Kanga kills them even if it contacts.
While I agree that Fighting types work as checks, they can not afford to switch in (or can only do it once) and are rather solidly countered by a variety of things themselves. So overall I'd probably favor a ban, after a suspect test that is.