I will tell you where he is VASTLY underrated: his intelligence. I am reading some of his quotes about different things like training,boxing and such an I gotta tell you his observations are spot on. I think Ali overshadowed him and if you look up some of Fraziers quotes you will be surprised at how observant he is.
He had only a rudimentary education and was fairly inarticulate ,certainly compared to Ali who tended to repeat well rehearsed doggerel.That obviously doesn't mean he wasn't smart as far as common -sense and observation went,similar to Liston I would imagine.
No one standing 5'11 and knocking out people left and right on the Heavyweight division should be considered overrated.