His inconsistency really was a massive flaw but it also made him a potential live dog in any fight he was in. His chin was up there with the very best. That Holyfield managed to drop him actually says a whole lot about Holy's underrated punching power than it does about Mercer's toughness, which was proved time and again against some of the hugest punching heavyweights of the 90s and early 2000s.
Mercer to me is right up there with guys like McCall and Chuvalo. His comp is certainly equal to McCall's and he's overall a better fighter, imo. Chuvalo of course has him on the comp, so this is what makes him, imo, probably the greatest iron-chinned HW.
I'd put him below McCall only because McCall has never been dropped or even remotely hurt. Chuvalo's a good shout, as is Tua.