Jack Black rarely really plays himself in voice acted roles. He really embodies Bowser in the Mario Movie, and even Kung Fu Panda he isn't really himself, so I'm not really worried for Claptrap.
As for the other examples, I don't think actors have to be young or yolked to play a character. Not that I see Hart as Roland, just that I don't think his size matters for this at all.
But yeah the movie could really go either way, there's a lot of potential but Borderlands could very easily be a franchised series so I do wonder about the casting from a different perspective.