Cant really kill or completely force out of lane since he can just lifesteal under tower, and it's insane to try to tower dive him unless you had 1-2 other people. If you teleport/roam to another lane to try to get them fed, he'll free farm his Q even more, or just take your tower in 1 push (since his Q still applies on turrets...even though they removed practically everyone elses skills that hit turrets but left the one that can literally 2 shot them.)
He just builds 1 tank item against you and then becomes almost impossible to 1v1 due to his sustain and Q damage, and the champions that can fight him at that stage are generally champions that get screwed over by wither the most. As people said, the only real way to win a game against a Nasus is to win the game before 30 mins, which isn't that easy if the enemy team is smart enough to just sit back and play passive/farm for the 5v4. The fact that he is basically "Is it past 30+ mins and you have a Nasus that at least some what knows what the champion does?" if yes then you basically win the game unless you make a mistake and force a bad fight etc.
At least with
they both actually have real counter play besides "win the game quickly". Sion only stacks health which is easy enough to counter with all the % health champions and items around. Veigar might have insane late game damage but is squishy and can easily be 1 shotted. Nasus on the other hand just forces you to send multiple people to fight him before he can 2-3 shot your turrets, which makes it really easy for the other members of Nasus's team to get another tower or Baron.