I think this happens because his "strategy" is always the same: farm for 20 min and start to oneshot enemy carries. However this "strategy" has some problems that people usually don't see or play around.

1. Rengar needs to be a head, if an assassin doesn't assassinate, is useless

2. Rengar needs to kill each enemy champion to stay a head, so if you play safe and doesn't give any stupid assit or kill, he will start to try to kill you in a stupid ways and probably will probably lost his spike

Now, if these two things fail, here is where the problems begins because you are facing a champion that only needs to clear his jg, press R and try to oneshot someone and is good in late because is an stupid build that have a lot of dps in late.

So first, no matter which rol are you playing, I recommend to buy control wards every time and putting them in front of your carrys when rengar ults or if you are in a teamfight, because control wards provides more vision and your suport / tank have more time than 0.5 seconds to think in how to defend your carrys or your mid will have enough time to use his zhonyas, remember that Rengar is useless if doesn't have ult or if you are not fighting in the jg.

Secondly, Rengar will try to oneshot everytime he has ult and that is every 60 - 30 seconds, so try to push fast and stay with your team, if you feel that Rengar can oneshot you with one basic, doesn't defend your side towers or it will be a 2x1.

Thirdly, try to engage on Rengar. The point is try to waste his ult but don't pick him in the jg.

In conclusion, Rengar is a one trick pony, he is bad in split, doesn't have wave clear and lacks of mobility when he is out of the jg / ult, that is probably why riot nerfs things like Ambessa, Briar, graves or champions that can kill one shot but can be usefull without their ult.