Is Kassadin ACTUALLY a late game monster?

I've noticed that a few of Riot's captions on their Kassadin nerfs/buffs would say something along the lines of this. However, from what i've played of him (maybe about 200 - 250 games), i t doesn't really seem like he's all that crazy late game. I've been fed as him (20/2 whatever you get what i mean) and i mean sure he can definitely burst down a squishy with a good amount of items, though it feels more like wittling them down. I usually have a difficult time fighting just about anything with a good amount of MR as him as well. When i think a lategame monster AP assassin, the first thing that comes to mind is Fizz. Kassadin doesn't really do that much damage compared to him. This is probably due to my lack of an understanding of the game and the champions themselves though... anyways, is Kassadin really that good lategame or am i just building wrong? Is there something im missing about what makes him strong lategame (If he is) as riot meantioned him to be when they last buffed his W?