Nunu is a champion that does not really have a concrete role in the game. But, it does not mean that he is broken. It only means that he is a jack of all trade. Nunu can either be a support at bottom lane, a jungler, a top lane bruiser or even a mid carry. He is that great that he can take all roles in the game except for a damage carry. He can go tank if he wants to. He can go support which everybody nowadays are playing him all because he plays all roles very well.
When you have your free riot points, make sure that you have Nunu on your champion roster right away. Nunu is best placed as a support. In fact, he is the top support in the present. He synergizes well with each and every carry he babysits at the bottom lane. He can stop enemies from ganking his carry and can buff his allies as well as himself. If you have problems with sustain, he can always consume minions to heal him back. Nunu also has one of the most powerful ultimate skills in the game but it does not mean that he is overpowered. His ratios are good enough to dominate the lane early, middle, and late game. There is no fault to Nunu or so it seems.
The only thing that sets Nunu from other support champions is that he has minimal crowd control spells. Although he can buff allies and slow down enemies in order to escape or channel through the ultimate to dish out heavy damage, he still lacks that powerful crowd control spells that Alistar, Leona, and even Nami brings to the table. This makes him a well rounded champion in the game. He is not overpowered and obviously not underpowered as of the moment.
Click here to know more about this