Vayne ult vs. Kha'Zix ult

Here is another situation in which kits are balanced incoherently to each other. I had already made a post wondering why Kalista spear stacks have no cap while Twithc poison does.

Why can Vayne have pretty much unlimited casts of her invisibility, while Kha'Zix only has two and has to evolve to get three? Back when he had damage reduction on his ult it made sense to cap its usage quite heavily, but now? Literally Kha'Zix ult only makes you invisible and gives a small move speed bonus. Vayne and Kha'Zix have ults that do pretty much the same thing, but Vayne's a lot better. Really, if you are gonna have two ultimates that do pretty much the same thing they should be balanced in pretty much the same way. Either nerf Vayne ult or buff Kha'Zix ult. It would only make sense.