rengar is new everyone thinks he is great and some think he sucks. alot of ppl say; rengar has no late game blablabla. WRONG! the real reasons why they dont have late game is because; they go jungle (srsl dont do this) they buy wrong items (pls dont make tanky that sucks -.-) and they dont use their ferocity well ( before u go in a fight build up your ferocity by spamming skills on minions/monster)
why the 3 inf edge? rengar his damage comes from his q. focusing damage on it makes it a great early mid and especially late game. the 75% crit chance makes it possible to kill even without your q.
but how much damage will i do? the damage is around 1000 for a normal q and 1500 with fero q in late game.

rengar has very high damage with q. (its your main damage whole game long) can escape good with ult in late game.
can last long in lane by using fero w. great special item.
tanky the first 3 seconds u jump into fight because of the w.(average 1-1 fight is 2-3 hits so the third 3 second wont come anyway

the 1 and only real con= ur getting focused the **** out of u. (just jump in later with q and everything will be ok.