Why are certain Assassins allowed to be so risk free?

Within the Assassin class, there seems to be two distinct types.

You've got the Rengars, Shacos, Talons** (to an extent) who do very poorly at range, and once they are jumping on someone, they are stuck in there for a while. The first two play very much like divers when compared to Diana or Reksai, minus the defensives they get.

Then you've got the Zeds, Leblancs, Katarina who can freely pick bad fights, but instantly retreat, or at the very least once they have killed their target they can just instantly remove themselves from the fight.

Why are the second class allowed to keep their huge reward when they carry barely any risk in comparison?

Assassins were always meant to be high risk high reward, but with certain picks, Zed I think is the main offender, there is just very little risk involved with his entire kit. No mana costs but better waveclear than most mids Has the ability to dodge any skillshot he wants in a fight with his shadow switch, which requires a huge amount of luck to hit it as he switches since there is 2 shadows, not to mention the iframe. Little burst delay since majority of his damage comes from the WEQ combo rather than his ult

But enough of Zed.

Why do you keep balancing a class as one when there is such a huge split between the two styles?