Well they both failed at their jobs when they were reworked. LeBlanc became a kiting mage, not an assassin. Adding a fixed burst window goes against assassin design. At least Zed's fixed time does not require his actual presence to occur. LeBlanc had to be within casting range the entire time and that was problematic.
Rengar became clunky, which is terrible on an assassin. They're meant to be fluid, quick champions and changing his Q to this awkward, half dash aoe cone just slowed him down immensely. It didn't fix the problem that the rework was meant to address which was him deleting champions instantly when fed because when reworked Rengar got fed, he'd still delete just in a clunkier manner as well as in an aoe so possibly deleting groups.
Katarina's rework was a wild success, as much as I hate to say it. They made her extremely telegraphed but also extremely mobile and having lots of options in most situations. She's no longer an extremely feast or famine champ hard countered by CC. They just have to be sure her numbers are in order.
Talon's W was slowed to give his targets a chance to breathe before they're killed. If you remember old Talon, could charge Q, E, and throw W and ult at the same time and the blades would essentially converge at the same time as your Q-empowered auto hit and the entire kit just came together in one, crazy burst. I'm all for assassins, but that's a bit silly. What Talon needs the most is for Riot to go in to -every- single bit of terrain Summoner's Rift has, by this I mean the walls, and fix any and all bugs that cause's Talon to jump in some stupid manner. I've died/failed to get a kill so many times because I E'd a wall and Talon decided to flip 3 yds to the right or something dumb like that. The W does feel clunky, so it would be nice if they could increase the speed that it comes back. Not necessarily projectile speed but the time it takes to start coming back, even by like .25 seconds. Overall, I think he just needs some animation smoothing and cast time changes, no real rework.