Assassins are not are terrible
Assassins at their core are not inherently toxic. They are most certainly annoying but they are not toxic. They are snowball/lead reliant and usually needs items or you to mess up for you to kill you. In order for them to be properly designed they needs to have a couple things
- need to be ahead or at least even
- immobile/dangerous to engage (E.G. rengar is immobile after he jumps into a teamfight)
- have a somewhat bad early game
- need outplay ability
- have to build glass cannon (or close to) (i might have missed a few points)
The issue occurs when they start breaking these rules. For example Zed has ridiculous poke and wave clear early game. Zed also is very mobile and can escape very easily. Leblanc is also incredibly mobile. When assassins start breaking these rules they get very broken very fast.
On top of this
completely rapes some assassins (and for good reason, it is supposed to after all). I didn't think the passive was worth the 1000 gold until i started playing rengar. If you are playing a champion that doesn't break those rules zhonyas will annihilate you. When used properly it will definitely save your life unless your team ignores the assassin trying to kill you, or you are so out of position that you will die anyway.
Assassins are not inherently over powered and just because you got instant deleted by a 10/1 Rengar does not mean zhonya's needs another 20 ap to be viable. If an assassin breaks one of those rules and god forbid more than one (
) that is when they start becoming over powered.