Tanks with Infinite Damage Scaling are not Healthy
Right now, there are two tanks with infinite damage scaling in the game- Nasus and Cho'Gath.
The issue with Nasus is that his whole kit is gimped to let him stack, and as soon as he hits a 50+% winrate he gets smacked down by the nerfbat. It is not fun to play against a champion that pops off to be an unkillable split push monster consistently.
Cho'Gath, on the other hand, stacks health, which is convert to a true damage "execute" by his ultimate, easily netting him 1.5k hp executes in the mid/late game, regardless of target, by just popping Righteous Glory and Stoneplate, then pressing R.
Neither champion is fun to play against, and games revolve around shutting that champion down rather than solid macro and micro play. I don't understand why these playstyles are allowed to exist when Riot repeatedly guts and buffs the assassin class for being feast or famine when entire games revolve around camping one of these champions so they don't hit a ridiculous breaking point.
EDIT: I'm complaining about poor design, not that they are overpowered. Also, I'm not using the Riot class of tank, I'm talking about champions that build primarily defensive items, sorry for the confusion.