In my mind, counter means beat. QSS just means he doesn't kill you in one rotation from 60% anymore. Fact is, he'll still safely farm and harass you until you're low enough to all-in again.
My point here is that if you throw gold at QSS you didn't spend it on armor. If he's that far ahead, he'll kill you without the ultimate and will just escape with it.
I'm not going to argue whether Zed is overpowered or not because I don't play him, and therefore don't know enough about him. However, I CAN tell you that I know enough to say he is by far the safest assassin and his counters aren't exactly the best.