The Cummings fight does fly in the face of the great chin / no survival instincts theory. Bruno was shut down completely by a single shot, but somehow managed to weather the storm and come back to win. Even on repeat viewing, it's still hard to see how he comes back from the condition he was in at the end of the first. Whenever Bruno's chin is talked about, you have to add the context that he went in there with a lot of big punchers over his career. Bonecrusher, Witherspoon, Tyson, Lewis, McCall - that's a lot of dangerous guys to have on the record. Fighters aren't supposed to take flush punches from that lot and shake them off.
Also I don't think Bruno would of survived McCall if it wasn't for the amount of spoiling he was allowed to get away with. He just barely scraped by Oliver by the skin of his teeth.