Mainly shock in my opinion, like launching at high rpm on a strong motor with good grip. You are applying a ton of torque in a short time period (shock) and that will break parts. I'm sure a high HP engine could kill one eventually too even if you ease into it. On the other hand, I know a guy with a lot of racing experience with them that swears when they are properly built they will last. He has had them in B-bodies that carried the front tires at launch with no issues. The OD helps with the short gearing on the street.
