Kai can either be an early game assassin or a late game hypercarry, not both

To be fair, she is pretty much a late game assassin as well, she can itemize zhonyas, and her w can do 600 damage on a very low cooldown. She fulfills to many different roles that many other champions reside in as their niche. Her early game damage is absurd as well. She is allowed to do too much because she has too many tools in her kit and there is not 1 point in the game where she is legitimately weak