Assassins are too rewarding for how easy they are to play

Most of them have access to stealth, a repositioning ability, or both. Most of them don't have skillshots, and if they do it's an ability with an extremely large hitbox (Kha'zix 'w'). And of course they have some of the best single target nukes in the game. And a lot of them also have resets on their jumps/dashes/etc, which makes it extremely hard to punish them after they murder your team. Oh, and I forgot, most of them also have access to abilities that heal them (Rengar, Kha'zix, Akali, Master Yi, etc). The amount of survivability assassins have in their kits is absolutely insane. And let's not forget about the massive buffs armor pen items got this year. TLDR assassins are way too easy to play and the class as a whole needs some serious gutting.

Oh, and I forgot. Most of them are also manaless. And most of their ults have extremely short cooldowns. And Riot makes it even easier for them to itemize cdr, since more armor pen items also have cdr.