Well in my opinion Lee (after all nerfs he received) is one of few very balanced champions (beside Vi) in game atm. His clear speed isnt very high or very low. Requires ALOT of skill to be effective in higher elo (lets not talk about low elo, even Soraka jungle can dominate in Silver/Bronze). Also, he is very easy to predict, you know he is gonna Q, ward jump and kick somoene into his team. You can bait him easily knowing that tactic- have done it many many times. His ganks are shit if you ward properly. His main ability is slow skillshot that can easily be dodged. He scales poorly into late game. Yes, yes i know if he kicks 4k hp mundo into his stacked team, he can deal tons of damage. But, think about it, if you position properly and dont stack on each other, he will just waste his ultimate (his ulti doesnt do bonus damage to whoever he kicked). And how often will Lee truly kick tank offensively? He will always try to kick high priority target. He has decent escape mechanism with his ward jump (and yes he was op with that before, when ward jumping gave him shield,but not anymore), but hey, he aint only champ with dashes... Ye i know, he has high base damage and his abilities scale well with AD. But u cant really get away with building full AD if u gonna be jumping into enemy team, so its fine. And i know people say "he doesnt use mana". To be effective, Lee has to auto twice after every ability to regain energy back.
Reason why even after all his nerfs, he is still popular, isnt because he is OP. Its because he is fun to play. And everyone hopes one day, they will manage to do that one perfect Insec kick. They can nerf his stats as much as they want, people will still play him, because simply ,he is fun.