Nasus has a 99% attack speed slow and movement speed slow (or atleast the att speed feels like its that much when playing any auto based champion vs him) and often runs ghost along that stormraider's surge or whatever it is, so slows are nothing to him if he can get even 1 Q off. He also has built in sustain with lifesteal and can 1shot people with his Q, he doesnt need a combo. On top of that he has a ultimate that increases his health and armor and magic resist (the latter 2 of 3 are with enemies nearby i believe) making him even harder to kill. If veigar misses that stun which is actually VERY easy since it only stuns on the outside, he can be effectively kited even within his own stun box.
Oh wait, i know what is going to come next "nasus slow just use quicksilver sash" guess what?!? Veigar stun can be removed with quicksilver sash as well. Veigar ultimate can also be wasted by using Zhonyas, talk about countered by items. Nasus just runs up to you and starts whailing on you with his Q that for every hit he has a passive 10% of its damage returned to him as heal not including the boost from visage...... And wait a second, nasus can Q towers as well every 1-2 seconds? But here, lets just complain that veigar can hit his delayed attacks on you because you cant dodge to save your life and dont know how to use a quicksilver sash/zhonyas.
That ap boost for the squishy veigar that once his stun is down is easy pickings if you just dodge the falling dark matter. You want an ez counter for him? Play ap blitz mid. That is if you can hit your skill shot. Late game you can burst for about 2.5k damage at least, more than enough for all but the tankiest of veigars and he will be unable to cast any skills to save his life.
Its funny how you only said nasus was the one that makes the arguement invalid, sion, thresh, and kindred are the ones i would have personally mentioned to try to kill the commentary with because they were just added in because they also had infinate scaling.
You dont want to fight any of them late game, its really that simple. Any of them can and will kill you with ease and have no qualms about it. The main one you should worry about is that constant split pushing Nasus that you need your whole team for because he really cant just be outplayed by one or two people very easily even with Quicksilver sash being used. With him Q farming and wiping turrets out without minions even being present, and healing 1/4th of his hp in one Q........
Please tell me more about this veigar problem when theres other champions just like him.