Reng main here.
The main reason why his ult CD is so high is so that he can keep his high burst and sustained damage without having to blow it.
Think about it for a second. Back when it was nerfed, jungle clears were so easy that Rengar could safely level 3 gank/invade and 100-0 you in a matter of 2 seconds. Obviously this is not the case now, but Chilling Smite is allowing Rengar to gank without 5 ferocity even - and keep in mind you can't buff a champion that already can be topping the charts if he's not an ADC.
What else. There's something different about his leap coding this patch, because before I'd always land a bola mid-flight and now it's more of a hit or miss. Maybe they are testing the waters, but if I were you, I wouldn't expect a Rengar buff anytime soon.
ALSO:
they took away fury generation during stealth, damage on Q, attack speed on Q and let his enemies know that the hunter was coming
And they added ferocity generation post-stealth with a movespeed buff, which is so much better for all Rengar playstyles, including assassin, ferocity Q damage is lower BUT regular Q is higher than pre-rework and a ton of attack speed that you couldn't neccesarily utilize before was changed to %AD bonus, increasing his burst even more.
you could bola them and always land the root, but now since its a skill shot, they can just dodge/flash it.
In most cases, pre-rework, if you had to cast empowered E on a target it pretty much meant you'd never get to them in melee anyways.
And a skillshot E allows you to generate ferocity at range if being harassed in-lane/escape more reliably by slowing pursuers from far away or without having vision of them. Not to mention being able to check bushes...