He's a good counter-jungler because you can go in and smite steal the other jungler's buff and you can Consume(Q) the other team's buff if your smite steal failed,.. The good side about being a Nunu jungle is that you have slows where you can support kills to your partners, but the bad side of being a Nunu jungle is that you have the possibility of not getting the f* out of there when you need to because of your movement speed,.. He's somewhat tanky though, which means that you have the possibility of not getting killed as easily when you engage, and in addition, you also have your Consume(Q) to keep you alive when there are minions around you,..

Pros:

You have enough life to support your team when you engage
You can deal a lot of damage when you engage in a team clash
You have a higher possibility of controlling the opposing team's jungle once you get enough assists and kills when ganking
You can easily help your team with your Absolute Zero(R) in a team clash when you engage

Cons:

You're somewhat slow if you do not have your Blood Boil(W) active on someone or yourself
You can run out of mana quickly if you do not have your passive ready when you activate your Absolute Zero(R)
Once you are counter-jungled heavily, you may not be able to comeback as easily if you're under too much pressure