A well made assassin will likely kill...

In about 2 to 3 seconds given being on even footing and targeting a squishy.

This allows a chance to react, to it use QSS and perhaps have an ally help win but the assassin should kill unless the target is massively ahead or getting help.

In other words Rengar is a bad design for an assassin, there is no good chance to react to his combo before it kills when he is about even or ahead. He is fundamentally a bad design. Once Rengar is killing targets in under half a second there is nothing much that can realistically be done about him in the game currently. The ! is not enough of a real warning since it literally shows up a max of a second before rengar is going to jump if he is going to jump.

Zed on the other hand is a reasonably good example of how an assassin should work. There is chance counter play, you can see him ulting on you and you can build to contend with him to at least some degree. This is not about being to strong or too weak but just that he does have a kit that can be buffed or nerfed as needed and will not be fundamentally broken to the point where he can not be contended with when having a bit of an advantage. Fizz is also in this boat where a few number changes can make him more balanced with realistic ways of contending with his damage.

This is not to say these champs are not frustrating to play against sometimes but they do have means to be contended with when ahead to at least some degree.

In this being said the new assassin item that is being talked about does in fact fit the design of how an assassin should work.