One shotting a squishy while being severely behind without any help whatsoever? I can think of what, two assassins that can do that? And not without farming enough for something like a Large Rod and Sorc Boots, unless you're Ryze.
Also being forced to buy defenses early as a squishy carry is a death sentence either way. Wasting close to 800 gold on a chain vest will hold off Rengar from killing you for so long, while your damage falls off and their carries get their damage items.
Twitch is equally a problem, but he doesn't do that 10 minutes in the game.
Also unless you're playing with a locked screen,
that little exclamation mark and a roar aren't "plenty of visual and auditory cues". He can just ult near you from the fog of war and you'll have what, a second to react and run? Not like it'll matter since he has a MS buff and vision.
He died 4 times after he left lane. 4 times, with no escape or buying any sort of resistances. Let's not pretend Rengar is squishy or helpless after he ults, specially when fed.
Like the whole rework tango Rengar went through was specifically so he couldn't do this. And now he's probably doing it better than he used to thanks to min-maxing.