There is a significant amount of counterplay to Rengar, but it's relatively difficult to utilize in unorganized play. One good ward is all it takes to deny Rengar an assassination for the next one to three minutes. If he has specced to be an assassin with Ghostblade and Mobility Boots, it's going to be hard to respond, but I can say it can be done. At this point, it becomes a mind game as to where he will snare and where you will move/flash.
If I'm not mistaken, I think what your issue is, is assassination pattern. Talon is usually pretty easy to deny an assassination or engage. Rengar tends to require hyper awareness of your champion, which isn't always a thing when you're looking around the map, since his pattern requires split second decision making. To be honest, I think Rengar's assassination pattern is why people choose to play him over other assassins. When Talon had his silence, it was the same as Rengar's pattern if you get bola'd. You can't do anything.