The game is unplayable if you have 1 weak lane against a Rengar.
No matter how down Rengar is, if you have a single weak lane he can just camp the shit out of it to come back into the game. Unlike other junglers, it is much harder to ward and play safe against Rengar since even if you can see him coming on a pink, with his speed boost it is often too late. The only counterplay is abandoning the lane and grouping which is not possible against strong waveclear. If you have 2 weak lanes or if your weak lane has low utility, the game's essentially over.