Most Kassadin's feed the enemy AP mid who has much stronger mid game presence then him, resulting in a 19-30 minute loss. Sometimes the team will carry said Kassadin to level 16+ and get him some items, at that point he becomes an extremely good janitor and kiter which usually results in viktory.
Or they are just a good Kassadin actually dodge the Anivias skillshots at mid and retaliate with Q's while maintaing a strong farm. This is very rare and I cannot pull it of against good players.
Or they end up aganst a Talon or Pantheon resulting in being 6 levels lower then their enemy and having achieves 15 less ganks. Then the team gets mad and reports them. VIDEO GAMES!!
Like every other mid AP assassin, he has the potential to snowball completely outta control.
Kass differentiates himself by being a complete wet noodle before he gets farm (rather than just before hitting 6 like Akali or Diana or someone like Fizz who can farm champs starting as early as 3) but also probably tied with being the most annoying assassin when fed as he'll not only oneshot your carries he'll also be able to just void walk out whenever (Diana and Akali have to go balls deep).
Hidetaka Miyazaki, Shinji Mikami Please come back Q_Q
the only bad thing about him which you wont like is that he is melee plus super squishy. but he does have a good escape with his ult. Also has troubles with mana. But other than that he can burst down any squishy in a second