He was fine before the rune system change. Weak early traded for infinite stacking. Now his stacking mechanism is like 3x faster, hes much safer in lane with the new rune system, and his ult not only gives health and magic dmg per second but armour, mr, bonus range AND 50% cdr on Q. Already shared my theory that the person responsible for this change was sleeping with a nasus player and they are probably married by now.
That said, a properly built mordekaiser counters him.
Enemy nasus was fed and ploughing through my team taking objectives non stop. As jungle morde I was equally fed, fortunately.
I decided to try 1v1 him in my ult. He was 1k stacks and I won. So, every teamfight I would ult him 1-2 seconds after he used his ult. Then I would kill him. The point is, even if I dont but survive the 7 seconds or even 5-6 seconds, he ends up back in the teamfight without his ult and out out position, also under 50% health at least.
Morde is a bigger raidboss than him if built right.
Thornmail (counters his passive and reduces his dmg a lot)
Blue-smite jungle item (300 health + aoe burn)
elixir of iron
TLDR: if you counterbuild enemy nasus you will be a bigger raidboss than him, or at least be able to make him useless in teamfights and waste his ult.
Also, i was blue smite, but red smite would make it even easier to 1v1 him.