If anything it has been mentally in the past, and he hasnt been pressured in any MMA fights since he jumped ship from the LHW div back in pride. Aslong as he's holding the hammer he's good to go, but when someone else is swining it he quits.
I also wouldnt rate being dropped in K1 and the likes either, since you get chances to recover, in MMA you eat punches/kicks/knees/elbows until a refs saves you or you tap to strikes.
And he doesnt have the cardio to play Cain's game, he wont have EVER unless he get's rid of some of his mass, so it'll be Overeem with a chance to win for one round, Cain will be in there for five if needed and he'll be able to win in any of them.
If this fight happens and Cain somehow ends up as the dog, i would drop ALOT of money on that line.