Let me explain as a Nasus main.
First off when I pick Nasus, I have a horrendous early game. I have literally 0 kill pressure, unless the other laner massive screws up or botches a dive. Not to mention because of said weak early game, I risk snowballing the other laner because for the first 15-20 minutes I'm just focused on stacking and staying alive.
Second, Nasus would be completely useless if he wasn't able to hyper scale with his stacks. Tell me what use Nasus would be if his Q wasn't able to chunk carries when he's been staying under turret and stacking the entire early game? Hint, he would be the worst champ in the game.
Third, he can be kited/peeled quite easily, even late game with 700+ stacks, especially if the ADC builds QSS and has a good peeler like Thresh, Janna, or Lulu. Also as you climb with Nasus, you'll notice that once ADCs start getting more skilled at kiting, it becomes exponentially harder for you to reach them and kill them in team fights, and there isn't anything you can really do about it, other than trying to flank. A diamond level Vayne is going to know how to kite me and kill me, and there isn't anything I can do to really "outplay" this because by his nature, Nasus doesn't really have much "outplay" potential.