He needs a window for his kills, so he can't just kill you in under half a second with half a build and no jungle item. And a window of actual vulnerability, which his changed W that wasn't reverted to its original state when it should have been along with his Q doesn't give.
While Zed is a problem for completely different reasons (namely that the game incorrectly treats a single cast of his Q as two separate attacks for Electrocute when it was specifically changed to prevent such things), the fact you can buy Zhonya's against him and/or have allies shield/heal you while you're marked adds balance to his ability to kill targets. Unless he's very far ahead, his kills should have a fixed time they take to pull off.
On the contrary, Rengar needs no more than a quarter of a second. Giving you and your team no time whatsoever to react with shielding/healing (especially now that both have been nerfed heavily), while there are also only two abilities in the game that can be placed around priority targets to protect them from his leap (Veigar's E and Poppy's W).
And despite the fact his Q was reverted to its stronger original state, they kept the massive damage-taken-based heal and cleanse on his W that was put on to compensate for the weaker Q. He shouldn't have that option to simply walk out completely unscathed even against an enemy with damage similar to his.
EditL Just going to add, his ultimate does not act as a window. There is no Stasis, Invulnerability, or Shield in the game that lasts even half as long as his ult. You'd have to make the boneheaded mistake of giving them vision of you to screw it up. Even then, this can easily be done from a bush without the need for the ultimate.