What's the point of diver-assassin style of champions?

They literally do what an assassin does, yet they lack the escape of an assassin or the innate survivability of a diver.

I am talking about / as glaring examples. Diana maybe has some form of AOE with her passive, a shield and the ability to yoink people with E, that could be used as a poor-man's Orianna ULT in a team-fight.

But Rengar is so badly designed(to quote our great Hash) that he does only single target damage(and a small AOE on W), he goes stealth during R and a big notice sign appears on someone's head, he gets cancelled way too easy by tank support and generally can't dive when more members are clustered together. Of yeah and he can't defend when getting jumped on unlike every single assassin in the game. Talon presses R, you pretty much die, Zed presses R you die, Fizz lands R you die.

You have to bush cheese and hope they don't protect each other or miss their CC.

On the other hand, your burst in unavoidable, 1 shots in half a second, little counterplay in a 1 vs 1. Again, bad design.

Makes you wonder why would anyone pick Rengo over other assassins unless you main him or you wanna have fun by 1 shotting in a braindead manner.

This type of champions in general are just bad.