He gives a solid performance in the Hangover films. Having just watched Birdman I am surprised to see him and I must say all the actors gave brilliant performance (Keaton Norton Watts Stone) including Galifianakis. I'd like to see him branch out from comedy...he's a hidden gem.
He has more range than people give him credit for. He's achieved fame as a comedic actor who plays a certain kind of character, but he seems able to do more than that. I remember seeing Bill Murray in Rushmore and realizing he was capable of far more than the goofy roles he'd mostly been given to that point. Who knows where Galifianakis' career will take him, but it's too early to write him off as just a comedian.
When you're given a role you either run with it or fall flat, Galifianakas has only fallen flat in what I've seen. You really can't be a good actor just some of the time.
In everything I've seen with him. I'm not a fan of films like the hangover to begin with, can't stand anything with Will Ferrel. Most modern comedies are trash.