Arguably Ali's most difficult opponent other than Frazier was Ken Norton- who had figured out Ali's style and nullified it. Not to mention, Norton broke Ali's jaw (however, I will concede that's actually a point for Ali, who obviously had great reserves of spirit to keep going).
Why did Tyson get knocked out by Buster Douglas?
Why did Tyson struggle against Evander Hollyfield?
Why did Ali lose to Larry Holmes, whom Mike Tyson defeated? Perhaps Ali was past his prime, just as Tyson by that point- certainly by Evander Holyfield. It wasn't age per se, but his shelf life in general with problems in his personal life. Took his focus out of it.
Ali had great footwork, but not the fast combinations and ducking and weaving as Tyson. You will not see Ali throwing lightning fast hooks and uppercuts like Tyson, who would land something brutal after slipping jabs as quick as anyone I've ever seen; Ali danced far away, light on his feet, but when I saw his fights, IF
an opponent could figure out his dancing, Ali got scored on a bit I would feel less than comfortable for a fighter who often kept his guard down. Perhaps because of his size, Ali couldn't land as many body blows on shorter opponents, but there you have it- Ali was only a head puncher. He did have a great chin, though, and if Tyson were forced to go past round 6, I think Ali's much better stamina would come in. Ali also tended not to get intimidated by fighters, that's another point that takes away from Tyson's main strengths.
But Tyson was lightning quick, much faster than given credit for- not as flashy as Ali, but more efficient. Tyson was prone to short bursts of violence, what I concede however is his stamina over the long run.