Hi
I think I've seen Liberace play what seem to me, to be some technically very difficult pieces, and I'm sure if you took all the Pianists in the world and were able to rate them, he would be in the top few percent. That said he wasn't as good as the great classical virtuosos. He wasn't a Jazz Pianist at all (as far as I know) so to compare him with that style is a bit unfair. Yes I've seen him play his fast boogie-woogie piece, which is great, but that's an arrangement, it's not improvised so it's not really Jazz.
However compared with someone like Richard Clayderman, who for a while was almost as popular, Liberace was a genius. I never understood why Richard Clayderman was so successful. There was someone who definitely simplified things! Likewise compared to a pop Pianist like Elton John, Liberace is in a different league as well, and I say that as an admirer of Elton John. He of course has an element of Liberace's showmanship and is a great songwriter.
All these things are relative, when talking about playing ability.
The video Pianoloverus posted of Dick Hyman (still alive age 98 btw!) playing Jelly Roll Morton is part of a wonderful program he did for the BBC (in the UK) in the 1980s called The Honky Tonk Professor. To a small audience he did a history of Jazz Piano from late 19th Century to Cecil Taylor in about an hour. It's essential viewing if you're interested in Jazz Piano, and I'm sure the full program is on YouTube. I videoed it at the time and have a copy on DVD now.
Cheers