There is 0 reason for infinite stacking to exist in this game. Champions like this, but espeically Nasus are incredibly annoying and unfun to play against.
But my main issue is Nasus.
Issue 1: You cannot push him into tower because he still sits there stacking. He will use his passive to get back HP, and be happy to farm under tower until he outscales you. So you cannot push against him because you will lose. You will either give him a safe farm area, or get ganked and he can slow you and kill you.
Issue 2: Far too much gank setup early game. I'll compare him to Vladimir or Veigar (both who scale very well, almost unbeatable at 3 or 6 items). Veigar has a highly telegraphed cage. It's easily avoidable, and is fair CC. Vladimir's only gank setup is his pool, which slows by 40%, but the enemy has to step into the pool. Not gonna happen most times, so he has about no gank setup, while Veigar has some gank setup/assistance, but only if the enemy doesn't dodge a telegraphed skillshot. Meanwhile, Nasus gets a point and click slow that slows up to 90% (not to mention it is a cripple, not necessarily a slow, so it also slows attack speed). This means he has an easy way to escape ganks, where as Vlad/Veigar, if caught out in the middle of lane, often will still die. Nasus's W slowing MS and AS means he often will just not die.
Issue 3: His Q costs far too little mana. If he has to stack his Q, then he needs to have a mana cost to it. The fact his most essential ability is on a 8-4 second cooldown (no CDR, with 45% CDR it becomes 2.2 seconds. That is just insane.
Issue 4: His Q allows him to not build any form of damage. For a champion that deals so much damage, he literally needs to buy 0 damage items. Again, if all his damage comes from his Q, then there needs to be a cost to spamming it. The facf that he can spam his main ability that deals 500+ damage on a 2.2 second cooldown while being incredibly tanky while building 0 damage items is insane.
Issue 4: His passive makes his early game far too easy. Vladimir has a similar issue, but he needs 45% CDR to be a problem, and his W and E cost health, so he can still get low. Nasus does not. He can be pushed in and be fine. If he pushes, even if your jungler is there, he can just W and escape (or E. Dunno which ability is the cripple/slow). If you are not pushing, and he pushes you and your jungler is not there, he can heal up and you cannot engage on him. His life steal+fighting is his wave+his Q means he does more damage at most points of the game (pre-5 or 6 minutes he loses, but not many champions can 100-0 hi that quickly). It feels as if he starts with a free Vamp Scepter.
I feel as if this kit is just problematic. Is it OP? Depends. If your team can kill him early and push in, he has meh waveclear, so he should slow down in his stacking. But this is only really applicable GM/Challenger, and Pro. In Solo Queue, most of the time your team is not coordinated enough that they can take him down before he is a threat.
I feel as if he is one of the champions who has pretty outdated design, along with Volibear and Tryndamere. Veigar could definitely have a rework as well, though I think he's less of an issue, because he only gains 1 AP at a time (Nasus can easily get 20+ stacks from one wave), and Veigar still has to build damage. He takes a long time to stack, and remains squishy. Nasus does not have these issues, along with having excellent in lane sustain.