The thing with Nasus is he can just dedicate all of his gold (and I mean ALL of his gold) to defensive items + trinity force, and still do ridiculous damage in team battles with his ultimate and Q. His Q scales completely independently from items, and his ultimate gives him damage by draining it from everyone around him (again independant of his items), and he also has a passive lifesteal. He is one of the few carries that can do this
This is why Nasus becomes ridiculous late game, and against certain matchups probably unbeatable. He can tank entire team with his ultimate, and he still does ridiculous damage with his Q, and he has one of the strongest snares in the game to catch up
Of course, this comes at a serious price. He has no gap closers, which leaves him incredibly weak to certain heroes early game, especially in lane (teemo, vayne are obvious counters). It also leaves him very vulnerable to ganks that are heavy in CC/Damage. Even late game he may find issues dealing with a team that has a lot of ranged carries + last whisper, as he will have trouble reaching them and not losing something like half his health
I just got out of a game with AD Sion that described this, Nasus still has a +1 on Sion in the fact that he can dedicate all of his gold to defensive items, and that he steals life without having to hit targets (AD sion actually needs to hit his targets to get health, Nasus gets it passively from his ultimate), making him weaker against kiting. Nasus snare is also a lot stronger in chasing, also not as good as Sion's in shutting down the enemy carry from dealing damage