What makes Nunu so hot and cold? (no pun intended)

What is it about Nunu that makes him the absolute best meta jungler sometimes, and the absolute worst other times? There are several other champions like this, but their tipping points are obvious. Assassins suck when tanks are prevalent, ADCs and squishy mages suck when assassins are prevalent... But I'm just not sure what makes Nunu such a "Great one patch, garbage the next" champion...
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