Both have arguments for and against.
Old veigar by all standard seems without counterplay at all with a much more reliable burst.
But
- he had to make a choice between farming and harassing. A veigar that wanted to have a lot of AP stacks had to give up on lane pressure and basically lose trades
- his manacosts were huge, and his passive hardly compensated anything
- his cooldowns were even higher and his typical build only included DFG for cdr, making Veigar insanely vulnerable after he used stun
- his AP total was lower than he is nowadays
- he was typically a burst champ and made the teamfight from a 5v5 to a 4v4 as he was borderline useless after unloading combo for quite some time (yep he can launch some Q's but that's short range and all he has)
- his lategame was actually rather average/meh as people got banshees and he couldn't burst ppl anymore. Which is not the case for current Veigar which is more of a teamfight mage with lower cd's.
New veigar is less punishable due to lower cd's and manacosts, he's more viable in lane, he's more "healthy" because you can dodge muh skillshot and not bother to get smart enough to counterbuild. He's much less of a carry one-shooter which made him kind of an assassin/burstmage.
So all hail the carry overlords and better remove all reliable ranged CC just like Sion's/Taric's because the poor adc's and stuff have only been metagame for all our seasons so far.