Why is Assassin balancing so inconsistent?

The biggest example of this I can honestly find, is stealth. Rengar and Pyke have to give warnings for it to be balanced, but Eve can be next to you all game and your only warning is basically a literal kiss of death. Shaco can pop out of no where and a single auto will kill you, with a 4 second Invisibility. Why in gods name do they not all give some form of a warning.Also no, Shacos Smoke in the fog of war doesnt count, because that comes down to the luck of having your camera there.