Veigar is not weak, but
BUT he needs changes, because he doesn't feel good to play at all anymore, because his kit right now is all about punishing horrible placement mistakes/ chaining his spells on his allies crowd control.
Veigar lost all his play potential. Against good players with good reactions, no matter how good Veigar is, he will not land his spells (E and W) by himself.
He needs an ally presence or a mistake of his opponents to do that. This is not the case for most midlaners, even CC skillshots aren't as hard to land (
charm,
chain... and these champs have mobility that Veigar doesn't)
Overall, Veigar is viable. He isn't in a good spot for a champion though. Unless you play in low elo where people have strong latency or are quite novice, you can not do anything rewarding on your skill. You land your E/W? No, it's not only because you did well, it's the opponent who did badly as well, everytime. And this is sad and not what we want.
PS : then why do I say he is not weak? Because his cage still brings a lot of utility especially for peeling, and his burst is scary.
Also, old stun was a bit unfair, but actually the fact it wasn't completely instant (cast time) brought a lot of mindgames : is he gonna stand still, move towards me or away from me? And sometimes against Veigar when you felt what he was gonna do (where he was gonna put the cage) you could actually not get caught by the edge. It wasn't as braindead as people think. Maybe with a very slightly longer cast time/ removal of warnings we could bring back that feeling , but right now it's just too damn long, and no mindgames anymore. Anyway we don't necessarily need to do that in particular, but we need to do something. Even if Veigar is viable.

means your ahead for the team fights.