Nasus and Veigar are different in what you want out of your team comp. One is a 3000+ HP tank with a 1/2 hp shot to adc's that you need atleast 3 people (min) to fight. Veigar is a cc heavy one shotting wine glass cannon who gets insta-killed by the slightest amount of cc. Where one is good at team fights (veigar) and the other is good at skirmishes (nasus). Both have crap early game but strong as hell late if they are playing at a constant rate and get what they need.
But my only problem with nasus is that he gets to late game super earlier than veigar who has to get to the 45 min mark at max to reach his where nasus just needs to get to mid and he can do the damage of a late game nasus super early when adc's are like 2/4 in their build and he feels like he has all items.
This is where his stacking is annoying, because even a nasus who is lane bullied can get super strong, not at the same pace but strong, in mid game. I feel like there should be a limit to how many stacks he can get but at a curtain point in the game, kind of like gathering storm. (this is coming from a non nasus player, but a veigar main)For example, he can only get say 150 stacks and stay with 150 until 10 min; and then can continue reaching his limit till 20 min. So that way he cant be super fed mid game and two shot adc's.
Same can go for veigar, but his stacking takes a lot longer and can be slower than nasus' stack farming and i personally think its fine but i can understand why people dont like it. The only time veigar can really get alot at one time, is if he hit a champ with all 4 abilites and kill them because thats about 9-10 straight ap that he gets. Where as nasus' is a bit safer, besides his laning phase, and simpler because he just has to kill minions/monsters with Q to get them and its pretty much just last hitting with added damage to secure the last hit.
Just an idea, but i dont know what his normal stack numbers are so just work around my concepts <3 