He is squishy, but he bursts the shit out of ADCs and other assassins with very little warning. The second you see that exclamation mark over your head, there really isn't much you can do. You could Zhonyas, but ADCs won't have it. You can't plan a counter-attack since you don't even know which direction he'll be coming from. Even if he's squishy, a decent Rengar player can take out 2 enemies in a teamfight and do significant damage to another. He'd maybe die in the process, but it's a huge impact
My problem with Rengar isn't so much his damage, but it's mainly his ult for me. I mean, his ult and his passive basically do the same thing except one gives ferocity and doesn't require you to be in a bush. And also, like I said, it barely gives any indication that he's there except the exclamation mark, but as soon as you see it, you could be dead and there's really no playing around it unless you've got a pink ward, which still isn't usually helpful because you don't know which direction he's coming from and if you just place it under you, he'll leap to you before it reveals him
So I think they should just completely rework his ult into something that makes sense. I think it'd be cool if his E and ult went together, like he could leap to a bola'd enemy and do blah blah blah damage and maybe some type of CC, like a brief stun maybe. But that's just me. He definitely needs some changes tho