I don't know what you are talking about. Katarina isn't overpowered at all. I'm sure you have heard this many times, but she is too shut down by crowd control to be extremely viable. Any commonly used Support or Mage can quickly end her ultimate which makes up majority of her burst.
Akali is actually pretty good at the moment. However, she is good in a way that is healthy and is not stupidly good. She is in a very good spot in my opinion since you can retaliate against her and she doesn't have any REAL crowd control that will benefit her when killing her target.
Fizz is pub-stomper. People complain about how he is overpowered yet you see him rarely in competitive play. Know why? His laning phase is terrible. Any half decent control mage can completely destroy him and other Assassins can use their mobility to get away from him since he will majority of the time initiate with his E. If he initiates with his Q, then just immediately apply crowd control to him and back off.
On the other end of the spectrum...
Rengar's snare isn't his problem really. It gives him really nice ganks, but it is how he ganks that is too good. Due to his bush leaping, invisible ultimate, he has so much freedom to gank fro wherever he wants that no pink ward can stop from where he ganks. You get the warning that he is there, but his leap is so long range that you have to no exactly where he is coming from to get away in time.
Kha'Zix is good because he can adapt to whatever he needs. He has one upgrade for crowd control, one for damage, one for increased mobility, and one for increased stealth. Kha'Zix has a bit of everything. No matter what they do to him, he will always be really good unless they rework him.
Zed is a little bit in the same place as Kha'Zix. His kit has so much synergy, damage, and mobility in it that it gives EVERYTHING an Assassin in ANY meta wants. There is little to nothing they can do to him besides just nerfing his numbers or reworking him that will make him shift out of favor. His ultimate is so great is because there is little you can do to stop him from at least applying the mark on you. Zed will always deal damage to you no matter what if he targets you.
Talon's invisibility was the opposite of his problem. His E was like a mini-Zed ultimate with an added Silence. It gave him more damage, unstoppable engage, and a crowd control effect that allowed him to pull off a full rotation before slipping away with his ultimate... or at least trying to. Now that that is gone, he isn't a real problem anymore. He is still an amazing Assassin if you can pull off that Q enhanced basic attack during the short slow his E gives, but he isn't overpowered.
, 
and 
(imo) have been left alone for ages. They even modified items specifically for ADCs yet they still complain about 
snare, 
slow, 
ult, 
invis etc.
Why are assassins taking the fall for noob adcs who dont stop to think about building a defensive item? Its their fault after all.
