As a Nasus main, he is easily countered. Typically, any moderate amount of coordination by a team with hard CC will shut him down. Nasus mostly hits his spike in the mid to mid-late game, not the late game. Teams have difficulty if they lack any coordination when fighting him and go in one or two at a time.
I don't even pay attention to my stack count after 500 but I start paying attention again around 800. Once I hit 800 stacks, it is a sign that the game has gone on too long! It is probably a close game and I didn't (along with my team) take enough objectives early to win the game. We are in the LATE game and that game has probably been going on for 50+ minutes.
After a certain point stacks don't matter too much. I do a lot of damage and can chunk people but the opposing team has probably figured out how to use their skills to limit me in fights. The games I lose the most often are the ones where I hit 900 to 1000k stacks. The next team fight probably determines the game and Nasus just stinks at teamfighting.