Only if he is OBSCENELY fed can he one shot an adc. Otherwise they have plenty of opportunities to get away, or to get peeled for by their team(especially supps like Janna, with her tornado, or Thresh can flay Rengo out of the air). Also, you can pop zhonyas easily as soon as you see him in their air as an APC if you have the reflexes. ALSO alot of champs have self peel skills like my main, Azir, or escapes built into the kit like Vlad pool, Fizz trollpole, and many more. Even most of the the ADC's getting "one shotted" have many escapes, like Ezreal E, Cait's net, Lucian dash, ect. And Rengar's bola, even while technically being hard cc, is a ROOT, so if he dives a ADC, especially as they are usually in the middle of their team, they can fight back, and usually have alot of lifesteal, and being in the middle of their team, can easily survive. Also, I looked you up, your UNRANKED, which means you dont have very much expeience yet. In time you will learn how to deal with Rengo.