as a whole, when you build champs AA, you have to focus on damage items, and not defensive items. Nunu being melee, needs to get in close to AA, which means he leaves himself unprotected, because hes been purchasing damage items. Ranged carries can get away with this because they are able to attack from the safety of distance. I'm not saying AA Nunu cannot work, but it's not as favorable as a champion that can do the same thing, but from the safety of 600 distance. I think Nunu works well as a chaser, because his slow is so powerful, and you can get some auto attacks in there. You don't really want to use him as a primary source of AA DPS imo.