And sometimes being lucky (or aiming a bit further back where your margin is greater...) pays off.You have to put all the factors in your favor. If you don't bad things happen. I've had arrow stop dead. Like 1" of penetration. It's a bad feeling! I've done it at least 4 times over the years. Which caused me to aim too far back and then more bad things happen especially if they step forward at the shot. Now I just aim for the Vital V and let'r rip. I usually either break the shoulder going in or the shoulder going out. They don't go far when they can't run.
I agree that heavy arrows and 2 blade single bevel heads are a good choice. I just don't think they're the only good choice, or that you ever have *all* of the factors in your favor.