He was nerfed to oblivion back in 2016-17 and now i will point some arguments that AP Rengar has counterplay and he should be viable again.
(But at first let's look at his scalings. His passive got deleted, w base damage is laughable; at some point your ferocity w deals LESS dmg than regular w.)
First, he scales pretty slow, he needs runic echos, predator boots and spellbinder to actually deal reasonable damage nowadays. So cutting him out of gold in early stage of the game makes him shitty champion to play.
Second, his combo is QEWW, so it takes more time to cast it than ad rengar usually does. I have played him in the past month and it is not that fucking hard to click zhonya or any ability that makes you invicible for a second. Building any mr tones down his dmg heavilly bcs he relies on ONE ability the whole game. It is still possible to flash out from him while he jumps at you.
Third, he is not teamfight-like champion. All what rengar wants to do is oneshot the weakest target and get the fuck out. Also fighting in open zones like mid, makes rengar (without ult) a fucking clown that wants to die bcs thats all he can do.
I like playing off meta picks and actually win, for example AP Ezreal mid, but some fuckers played him hybrid so he got nerfed. I really hate that Riot avoids to touch the problem and jumps around it. This company systematically streamlines every aspect in this game. They say they support alternative ways to play the game but 5 minutes later they are doing the exact opposite. For example AD Fizz was quite popular back in 2015 and he got obliterated. But it is okay for AP Fizz to one shot people with just one item now.