User blog comment:Deshiba/No counterplay/@comment-24952680-20140518103355/@comment-4022742-20140519133710

And really, Kassadin's Level 18 burst consists of 680(+220%AP(+2% Max Mana)) with a once cast Rift walk. Assuming, and  without runes that would give level 18 Kassadin 3190 mana. So: For an assassin that burst is kind of laughable, considering ;1435(+370%AP), ;950(+107AD(295%AP)), ;1325(+375%AP(+435% Bonus AD)) and many more.
 * 680(+63,8(+220%AP)) no stacks
 * 900(+191,4(+220%AP)) max stacks

Comparing Kassadin to other assassins is kind of a moot point, he has become more of a "Stick to the target" kind of assassin. He has to utilize his ult and nether blade to the fullest in order to kill anything by himself. The mentioned in-damage-out strategy has it's merits but won't net you a kill against a careful opponent, or one that has more burst then you do.