Why are people complaining about Nasus's kit?
According to lolalytics Nasus has a 56% win rate with Kleptomancy and a 50% and below win rate with all other keystone runes. Op.gg has similar stats.
A 50% win rate and below champion is not a problem, yet most of the Nasus complaint threads I've seen talk about how strong his stacking is, or the Q CDR on his ult, or his Wither. Sure those are strong abilities, but if they were the reason Nasus is so strong right now his non-Klepto win rate would show a problem.
Let's focus on the actual problem here, shall we?