Well, I'm not sure if it's the point you're making but she's had a low win rate for a long time. Even back when single burst assassins were ruling the game, LB had a low win rate.
Oddly enough single target assassins almost always have a low win rate, regardless of their current state of balance. LB's win rate actually isn't very strongly tied to her power, she had a 45% win rate before the removal of her silence, she has a 47.5% win rate now, she was definitely stronger when she had her silence though.
What do you do against Zed? Every time I see a LB complaint thread I wonder about Zed. I get that your primary concern is LB's ONE advantage over Zed, she has an ultra-reliable combo to bully laning phase.
Can't more than one champ in the game be lacking counterplay or unbalanced? Just because Zed's a piece of shit it doesn't mean LB can't be one too. Honestly I've always considered them too be two sides of one coin, they have most of the exact same problems.
Zed has a better escape with his shadow (as he can go back and forth, and his shadow can also use abilities)
LB and Zed have literally the same escape pattern, they both leave a mark that they can go back to on the ground. Neither of them are better at escaping. An argument can be made about Zed being better at getting into fights but I'd say that's evened out by LB having a longer engage range for fights.
Zed has an easier time cs'ing with his passive (especially if you push him under tower)
While Zed has a harder time CSing under a tower he's a lot easier to poke off of a wave due to him being melee.
Zed has waveclear (LB's only waveclear is her W, if she waveclears with it then you're free to pressure her while it's on CD)
Zed realistically has to use his W to clear a wave, at which point you can pressure him unless his ult's up. While you may not be able to pressure Zed if his ult's up you also can't pressure LB if she Ws into a wave and has her ult up as you risk taking a Q-R and possibly an E based on position.
Zed has a better all-in with his ultimate (he can now switch places with 2 shadows)
Zed all ins better, LB doesn't all in. She almost always has a range buffer to abuse that Zed just can't.
Zed is more useful late game (he builds AD so he gets to split push, he's more useful taking towers, dragon, baron, and is still useful in fights once his abilities are on CD).
That's somewhat questionable. Zed does the things you listed better but LB has her advantages late game too. She's got a higher immediate burst than Zed and can often instantly remove a squishy from a fight more effectively than Zed. She also has some capacity to peel with her double chains, which is something that Zed can't do at all.
Zed doesn't even use mana, let alone just not on his ultimate.
Which I'd say is one of the biggest design oversights that Riot has ever made.
None of this is to say Zed is OP (in my opinion he is, but that's not the purpose of this thread so I won't go in to that). It's simply to say that if you're fine with him, what's wrong with LeBlanc?
I'm not fine with either, and considering how high their ban rates have been over the last 6 months or so I'd say a decent percentage of the community agrees with me.
She has meaningful weaknesses:
Lacks waveclear
Difficult to CS with
Which are both mitigated by W max
Weak in teamfights
She has better options in teamfights than most assassins. The only one I can think of who's better in fights is Kat, but that's a whole issue in and of itself.
Easily countered by a Banshee's Veil
This is kind of the "Zed is weak to Hourglass" argument. It is true, but assuming everyone always has access to a counter item isn't always fair, especially since LB is rarely forced to deviate her build path.
Poor duelist
This is a weakness of the class. Every assassin should be a weak duellist, the fact Zed isn't is more of a testament to Zeds lack of balance than any thing else. It's kind of like if I tried to claim S2 Ez was balanced because he was weak to burst and S2 Graves was more broken because he didn't have the same weakness. I've picked something completely separate to their role and compared them on it to claim the weaker of the two was actually stronger. I'm not claiming that LB is stronger than Zed but I think this comparison shows that you should only compare champions based on the primary characteristics of their class.