The problem isn't assassins being underpowered, its them not being able to carry.
They're assassins, not ADC's or fighters, they don't dish out consistant damage. As for those you just listed, besides Zed they're all very high-risk high-reward assassins, unlike champs like Leblanc and Katarina and whatnot.
Katarina is a carry assassin. That's why she has a higher win rate than most.
But, seeing as Katarina is still AP, she still has a bit lower than 50% win rate as she can't take towers easily(unless she builds AD which is rare). Though she can also just be walked away from when she ults, or CC'd, or just plain out Zhonya'd or Bard ulted.
LeBlanc is super safe. However, due to her only being low-risk low-reward, and AP, she cannot carry, meaning she relies heavily upon her own team. Hence her low win rate.
This is most likely why AD assassins have higher winrates, because they can take towers and they can help carry. Though unfortunately those listed have no way to get out of a fight. LeBlanc can dash in, take someone, then dash out and that's basically her kit gone for the next several seconds. However, Kha'zix, Rengar, etc., are all AD so they can still dish out fairly consistent damage. Though unfortunately they still lack the consistent damage of say, an ADC, so they can't carry their games. As in, they can burst a target, but then they're just a few hundred damage to this target and that target until their cooldowns are up. They're also extremely squishy and melee so they get focused and bursted hard after they engage.
Though something I do have a problem with is how you specifically avoided Talon's win rate. He has a huge win rate of 54.11%, and yet he can still carry. The sole reason why he has a higher win rate. His Rake and ultimate are huge AOE damage, plus he can just walk out with his ult after to ensure he doesn't die unless he gets oracle'd or pink warded, or possibly just CC'd before he can ult. His Rake is also on a low cooldown and he's AD, meaning he can carry through team fights, as well as towers. This is why his win rate is so high.
- **47% WIN RATE: ** One of the most mechanically challenging champions ever released, requires exceptional game knowledge to get even fed enough to be relevant.
- 48% WIN RATE: Rendered useless late game due to broken AP items like 
- **42% WIN RATE: ** Need I say more?
- Horrible laning phase for how mechanically challenging and high risk low reward he is
- **46% WIN RATE: ** HAS to snowball VERY EARLY to even be a threat come mid/late which is incredibly hard to do.
all above 50%
is above 49%
