Nunu's power as a champion is directly tied to the power of the objectives in the game. Rift Herald is an extremely binary objective that gives your team map pressure and gold with a single successful gank, and Nunu can solo it on spawn without taking much damage, leaving him more than ready to crush top lane right after with very little warning.
The main issue is that players are just flat out refusing to adapt to Rift Herald. If you see the enemy team pick Nunu, you should be prepared to collapse on him when he's taking objectives. You should run Teleport to deal with Herald. Honestly, you should do that anyway. The fact that Herald is obviously a powerful tool that if left unattended can give one team an insurmountable lead from very early on practically demands that you run Teleport, especially as a bot laner.
Nerfing Nunu's stats is fine, but until people actually start playing appropriately around Rift Herald then Nunu will continue to be a very powerful champion.
Oh, but his win rate has gone up recently. That's because Cinderhulk got buffed, obviously.