Hank has had character development in this franchise, especially in FIRST CLASS and DAYS OF FUTURE PAST, and he definitely has a character arc, both related to his appearance/bestial nature and his feelings about the X-Men and Xavier's stewardship of them. And no, his arc is not as well drawn or developed as Xavier, Magneto, Logan, Mystique, etc, and there's a reason for that...he's a supporting character in the franchise. In some ways he's a foil to Mystique and he essentially serves as Xavier's conscience. But he's been a key character in this franchise nontheless.
And I get that people have an issue with him changing back and forth, but that element actually does add serve to add more depth to his character, his origins, and his relation to the world at large, and there's been a gradual evolution of both his ability and then his willingness to become Beast. The serum isn't just a serum, it's something he has to constantly refine and evolve to stay "human". And then becoming Beast isn't just about turning blue and furry...he seems to become almost feral, to the point where he can lose control. It's not just about his appearance. It's about the fact that he more or less goes from a mild mannered fairly gentle scientist to a more violent creature, which we've seen glimpses of.
Beast was never the one who accepted his mutation outright. Even in The Last Stand he's tempted by the ability to change back to a human, and you get the sense that this has been a lifelong struggle for him.
He hasn't been spot on in terms of his personality, but I do think they've gotten a lot right with the character through four movies.