Shh, you'll piss off all the Kha'Zix mains who can't W mid-air anymore.
But honestly, Riot needs to decide whether they want him to even have the ability to E mid-air anymore.
If they think he SHOULD be able to E mid-air, then why make it so difficult to execute every time? It needlessly increases the skill-cap of the champion. Why is this a bad thing? Because, just like Riven, it means Rengar can be garbage in the hands of most players, while utterly broken in the hands of skilled ones. This will prevent Riot from nerfing him due to stats showing a low win rate, meaning those few dedicated players will have a completely unfair advantage. Making Rengar less difficult to use would allow him to be easier to balance, since Riot wouldn't have to compensate for that very small percentage of players who have completely mastered him.
Of course, most people who main a champion do take pride in reaching that level of champion mastery, and don't want everyone able to play the champion. This is still the current problem with balancing Riven: People who main her have worked so hard to master the champion that they would rather see her gutted than have that aspect of her removed that makes her so difficult, even if it would result in a more balanced champion.
Both sides of this argument have valid points, and honestly neither approach is the correct one; leaving the champion as-is will leave the game less balanced, while changing the champion will devastate those who have mastered that champion.
Going back to Rengar, if he SHOULDN'T be able to E mid-air, then they should hurry up and remove it instead of letting it linger like an open wound; it either needs to be properly stitched up, or amputated quickly to prevent infection. Leaving it with a smiley-face bandage and a kitten "Hang in there!" poster will make him a subject of imbalance.
As of now, if a player can actually execute the E mid-air combo through all its clunky interactions, then Rengar is considered obnoxiously strong. If the game decides to be buggy when he does the combo, then Rengar will fail that current mission, making him feel weak.
TR;DR: Rengar is super clunky, to the point of executing his combo correctly feeling like blind luck. Either make this ability smoother, meaning Riot doesn't have to balance based on the randomness of the ability working correctly, or remove it entirely, meaning Riot can apply power in other places.