His ganks are decent. Not the best, but decent. His armor shred and wither can be pretty nasty. The best thing about him is how he can just obliterate turrets. If he wills a lane to end, he will end that lane. Wither the opponent, and they helplessly watch nasus plow right into the turret.
His ganks are very strong if your lanes already have hard cc, kills/blown summoners are usually guaranteed. He scales very well with gold-efficient items like aegis and locket, which is a valuable trait for junglers. He functions well as both a utility and damage threat late game, which is rare in junglers.