Like Flagg said, teams that shut Taylor down said they "forced him to be a QB", meaning they didn't let him get outside the pocket. He's at his best when he's on the move. With a a pocket passer you can attack the pocket and not worry about them escaping pressure. With Taylor they didn't attack the pocket, rather they contained him, which gave receivers more time to get open.
With McCarron, you can bet opposing defenses will attack the pocket with a variety of schemes and blitzes. This means he'll have less time to throw, less time for receivers to get open.
I'm not trying to defend Taylor... My point is that McCarron (or whoever) is back there, won't be facing the same fronts and types of pressure that Taylor did. Receivers won't have as much time to get open.