He's done comedy a lot more than you think. There was absolutely nothing threatening about him in Bad Grandpa. He was basically the lone bright spot in the badly directed movie "The Gang that Couldn't Shoot Straight". Wasn't threatening at all in The Joker where he played great opposite Phoenix's psychopath.
Backdraft, Joy, The Intern, as Monroe Stahr in The Last Tycoon, Marvin's Room, and so on. Did you not see any of these?
He also brought depth and quality to non mobster "villain roles" such as the abusive step father in This Boy's Life and the racist diving instructor in Men of Honor.
Even in Flawless he and Hoffman gave A level performances to difficult roles to pull off. Ebert's review concluded with this:
"De Niro is a great technical actor who may have been attracted to this material because of the chance to play a stroke victim. His performance not only gets Walt's symptoms right, but also shows sympathy for the man inside. Hoffman, who played the pathetic man who made dirty phone calls in "Happiness," shows he's one of the best new character actors, able to take a flamboyant role and find the quiet details in it. Too bad they're stuck in a jumbled plot, but as an odd couple, they work."
If you think he's a one trick pony it's because you've only been interested in watching the movies he does that trick.