Lee Sin is OP but you can make a trainwreck out of his kit if you use it wrong.
Especially lane Lee Sin, you have to pay attention to your energy management. Don't just blow skills for no reason and forget to auto attack. You may laugh, but when you have no energy to Q-Q someone that's 10% HP you'll want to quit life.
He's all about using his OP abilities at the right time. It's not super hard but it is important. He can't straight up out trade someone unless he lands his Q, and if you are fighting a sustained auto attacker you want to save Safeguard to escape.
Since they nerfed shielding minions he hasn't been played top lane very much, but if you can farm enough for an early Tiamat and bully your lane, you can snowball pretty hard.