Veigar's ult shouldn't deal more damage on health missing.
Seriously it's capped at 33% of your remaining health and it's single target, at least with cho you can kite him. before you give me that "Oh build Hexdrinker and he's dead" "Oh save flash for his wall" Sometimes Hexdrinker wont be enough to stop all that extra damage and sometimes people will protect him like crazy
*no Veigar isn't broken, it's just his ult is just incredibly strong and difficult to deal with if you are a champ that doesn't have untargetable abilities/Items