You completely nullify his passive after his first attack. That is pretty significant, even though it may not seem so. Also, his ult serves no function at that point if you ever fight vs him, which is important if you are someone who is trying to duel him (which you should never do as you will get annihilated more than likely).
Then he either evolves W and just pokes from shadows + AoE for the passive procs; or evolves R and uses the stealth reveal to his advantage by reducing incoming burst damage.
You don't get to choose if you duel him or not, because of his E range he decides that one for you.
Kha'Zix does actually get screwed by CC (mostly because he has difficulty sticking on a target), but building tank helps counter some of the focus fire and he can always escape or keep the chase going if his team nets him an E reset.
He can't really use much Unseen Threat if he can't go invisible or duck out of vision. And if he can't do that, he can't deal as much damage (Q is his main damage source, unseen threat is his other; R is utility, E isn't primarily used for damage, W is relatively long cooldown and ow damage compared to other spells)
Unless his ult is guaranteed Unseen Threat proc, at which point I'd kind of like to see that go away.
Have you actually looked at the numbers? Unseen Threat is there for the slow, the actual damage is miniscule. It reaches around 100 magic damage with no m-pen when your Q does like 400+ damage with a-pen. And then by end-game, it barely scales to 200.
Making his ult give him a guaranteed Unseen Threat procs... unnecessary. It would make killing isolated defenders on points even easier.