I would say strength. There are a lot of guys who are much stronger than him. Cormier for one is stronger and can match his wrestling.
How good is Cain off of his back. Wrestlers are traditionally very uncomfortable off of their backs. Werdum could be a nightmare for him.
His standup is just ok. He moves like a robot. Luckily for Cain, most of the elite strikers in the division can be taken down.
Cormier stronger than Cain...? Not so sure about that.