I don't think he is OP or UP. He is very annoying to lane against and whether or not he is countered depends more on your team comp than if you win your lane. Because you almost always tie or lose vs a Nasus unless you get some early jungle help. The thing is that if you're AP Nasus will stack magic resist and you can't kill him due to his in build sustain. If you're AD he will get a lot of early armor and you won't be able to kill him. On top of that his all in potential is very good.
He falls off later on as well as gets stronger later on. He falls off because later on he's rather easy to kite and thus often being more useless than other solo tops like Sion. But he gets stronger because his Q is stacked insanely high and if he gets the up time he is just doing too much damage.
I never really though of this champion being a healthy design or fun. He is boring to play as and boring to play against. Having only a skillshot you can't really miss makes this also one of the easiest to play champions in the game.