Rengar doesn't really fit the Assassin "meta" that Riot is going for, with delayed damage. He also doesn't really fit thematically with being an Assassin.
Rengar, was, and still is, too a degree, about jumping in, out of stealth, and inst-gibbing a carry as fast as possible. Which is unhealthy. Its no fun for a carry to be deleted without any time to react, and its no fun for Rengar players to be shutdown half of their games, to allow for the games were they can insta-gib squishies. That resulted in Riot trying to delay Rengar's damage, by making his Q no longer instant. However, that really was just broken, especially with the buffs Bone-tooth gave. Realizing that slowing his burst, but giving him more damage wasn't healthy, Riot nerfed him. Still, he wasn't fair to play against. Part of that, is his kit. Rengar has no escapes in his kit, its all about diving in. He also has hard CC, something you don't see normally in Assassins. Leblanc is the only real exception, but she is half-mage, so that makes up for that.
Thematically, Rengar has some bad fits. For example, his old passive. It used to be that you could stack it off killing the same person over and over again, which Riot changed, partly because, it just didn't feel like you were a hunter searching for the strongest prey. You were more just beating up the weakest link, over and over again. Then there is the Assassin play-style with hunters. It can be argued that hunters are stealthy while taking down prey, but the dip in, dip out, is not how hunters work. It is much more thematically fitting for Rengar to be a Diver, than an Assassin.
To summarize, Rengar's current kit and thematic don't fit the Assassin role. He will likely never be balanced as an Assassin, and it is pretty likely Riot will either rework him to be a better Assassin, add an actual escape, remove the CC, or up his base stats, change his damage a tad, and make him a Diver.