Lets take a look at the weaknesses of a nunu support:
Lacks hard cc, you can pick a few champions that are cc machines (Alistar, Cho'gath, Naut) to pick up the slack.
Lacks a heal or a shield for teammates. Further, the downside of him relying on comsume to keep up his health is that he is stealing cs from the carry. That is only okay when there is no friendlies in the area. Aside from that, you do not steal cs.
His ultimate is hard to land. It takes good positioning, a spell shield, a very distracted enemy team or an enemy team lacking any cc that can stop a channel.
His passive lets him cast spells for free
His q is a very nice anti-dps tool
His blood boil helps increasing the dps of friendlies.