I'm not being sarcastic nor trying to start an argument, but I can't seem to find any down sides to Lee Sin's kit.
He has great engage and disengage, clears camps fast and safely taking next to no damage, built in shield and sustain, can duel like a God early game, extremely mobile among the most mobile champions in the game, has great burst damage, etc.
Where is this champion's downsides?
He falls off a little but it doesn't nearly compensate for his early strength.
He does have one weakness I use to counter him almost every time though -- being banned.
Lee Sin's weakness is that he is balanced without any particularly overwhelming strengths outside his earlygame scaling and occasionally displacement ability. He is well rounded but doesn't serve enough functions to actually get much done without some help or some sort of advantage. He is reliant on many things to function but can adapt to many things as well despite not being terribly useful on his own.
Lee Sin's weakness is his lack of outstanding strengths.
Lee Sin's weakness is his dependancies
Lee Sin's weaknesses are less apparent because his actual strengths are not that strong. He simply has many of them.
Lee sin is a fighter assassin or just a fighter if you want to go by newer standards or something like this if you want to go by even newer standards. Anyways, Lee Sin is the sort of thing the game is sort of counterbalanced against. Lee Sin is a balanced champion in the sense of what he can do. He probably seems op as shit to those who mostly play imbalanced things
or for the sake of avoiding confusion lets call lee sin symmetrical and some other champs asymmetric
Symmetric champs are champs who's attacking and defending ability are about equal and they have some utility
Assymetric are things like tanks or adcs who need someone else to help them with certain things.
In almost every game including mmos symmetric types will be strong as shit earlygame but as peoples builds get stacked the symmetrical types tend to get severely outperformed in a way that really shows especially when its no longer a 1v1.
Try actually playing Lee for a bit and you'll very quickly figure out what his weaknesses are.
he falls off late game also he has a skill shot he's semi reliant on. im sure there is more but I dont play him myself.
He is always oom kappa123. Seriously doe, he really has no weakness other than player making mistakes and being bad, same case as some other champs and they are all hated for that same reason 
etc.
He has a high skillcap, which for literally any 
other 
champion would mean getting the crap nerfed out of them.
He's heavily reliant on landing Qs in most scenarios which is fairly easy to dodge if it isn't point blank or you're CCd.
He can't handle heavier fighters at all on his own.
All of his mobility spells are interruptible (as darius I can apprehend him out of his Q or hop).
In order to have meaningful DPS he cant spell spam, he has to weave autos between spells. This ties in again with the fact that if he isn't fighting a squishy that he can just full combo he's going to suffer heavily trying to auto between spells for any real sort of DPS potential.
He falls off pretty hard. Late game he amounts to "can I make an insane play?" If he doesn't make an insane outplay, he's pretty much just a shitty fighter who can neither tank all that much nor deal any meaningful amounts of damage.
Lee Sin is vulnerable to CC, needs some planning and skill to play, and isn't the greatest late game champion. Not that he can't get into late game at all, but if he doesn't excel in early-mid game he won't be 1v5 solo carrying late.
This isn't to say he has his VERY strong spots, but you can't just play Lee Sin and expect to be a god (fist) on your first game. His clear is easy, but he takes planning and strategy for ganking and knowing what to do.
