What I think is the problem with Kha'Zix
Is that unlike most assassins, Kha'Zix doesn't have reliable assassination capabilities. His main source of damage is tied to a passive that basically does nothing if the enemies are grouped together. Compare that to assassins like Fizz, Leblanc, Zed, Katarina, even Kassadin. They all have reliable burst damage if ahead, some even being good when behind. Unlike them, Kha'Zix isn't good ahead or behind, unless the enemy just doesn't group