One of the most common critiscisms about him, is that he can't take a punch!
I believe that Frank's biggest problem was his stamina, the fact that he was too muscular & painfully slow. He didn't know how to react when under serious pressure, hence he would drop his hands & allow his opponent to take pot shots at him, that would eventually lead to the ref having to stop the fight.
If you look at his losses, they were all TKO's when he was tired & he was never involved in a fight where he suffered multiple knockdowns.