It has more to do with size and durability than skillset I think. I don't think he's suited for a fulltime RB. Some FBs play RB and TE, but that's not even a good fit because his skills skip over the main role of a FB. I've always be comfortable listing him as a TE when not playing QB (thankfully, wasn't a fan of him as a fulltime QB but a huge fan as a role player)