He's pretty well balanced but super frustrating to play against. The reason being you can shut him down super hard early, and he can still be very powerful by mid game. The reason for this is that his stacks work purely off last hitting, which is a slow process early game, especially when he's being hit hard. But since damage also scales with items, and level, as the game progresses, and even with a few Q stacks, clearing waves of farm and JG become relatively easy mid game. Just like how non JG champs can't jungle early but can plow through a jungle by mid game.
So what happens is he gets shut down, but then can get TONS of stacks after being left alone for only a couple minutes mid game, and that is SUPER frustrating to play against, and that's assuming you did shut him down, which can be hard, but is certainly do-able with the right champs.
Add to that a super burst squishy meta, and a champ that can build 95% defense, and you can see where it just compounds the problem. He doesn't need to build enough offense to kill squishies, his stacks do that, meaning his defense is going to get where it needs to be that much quicker.
As far as does he win vs lose. I think that's pretty close to balanced, he's pretty strong at lower tier games where I play, but I think that falls off in more organized ranks. So it's not so much does he win or lose as he is just really frustrating to deal with from minute 1, to the nexus exploding.
Of course, there are other additions to that, he has so much backup styles of play, he kills turrets super fast making him an amazing objective killer, he is super strong 1v1 making him an extreme split push threat on top of that, so even when losing he can be really difficult to manage.
All in, again it's just a frustration to play against more than "is he overloaded." People are forced to play his game instead of theirs, the game becomes about "manage the nasus or else" and that's not fun for anyone.
If you add that to a full AD opponent (for example) stacking armor is GG, that's on the team that did it though, which sadly seems to be pretty common. I'm surprised we don't see more off main tanks like Rammus and Malphite, or even bruisers in this meta just to counter these things. I definitely hate when I get early pick (who doesn't) but especially so when my team stacks all 1 damage type and I had no way to prevent it.
Just my 2 cents.