It's quite alright for champions to have different ways and magnitudes of creating opportunities. In similar sense, there are champions who can utilize those opportunities very well. From what I've noticed so far, is that champions who are great at creating opportunities get their raw power reduced significantly.
While Lee Sin is an attractive example, I'll use another:
Blitzcrank has a very strong tool to create even scenarios into advantages. He can do this from considerable range with a good deal of safety as well. In response to this, Riot has nerfed his tankiness significantly, so that he runs more risk should he engage on the wrong target.
The same counts for Lee Sin. Riot took away his W resistances, lowered the shield duration from 5 to 2 seconds, removed his AS slow completely (which was one of the things that gave him some much needed love late game), lowered Q damage in base, scaling and %HP, lowered cooldowns across the board, reduced Iron Will duration, reduced his AS, etc. They basically removed all of this so to compensate for the opportunities.
On the flipside there are champions who have next to no way of creating opportunities but can still overburden you with stats. A good example of this is Nasus. When she reaches an item and stack threshold, you can only pray that you have the kit or comp to stop him. He doesn't have any initiation or outplay potential other than Ult baiting, yet he can lay the law down on your towers and squishies.
If you bring this together, you'll see that Lee can't win from Nasus in a duel and can only run away. Nasus can not catch Lee for the kill, but he can put pressure on objectives. Strengths and weaknesses, it's how league works.
On to the argument of everyone being incapable of dealing with Lee, that's bullshit and you know it. If that was true, the champ would have convincing winrates in all levels of play, including competitive. This isn't the case, though. He's just popular.