The issue is Lee doesn't excel at anything. There's usually always a stronger pick you can take for what you want to accomplish, because he's a generalist pick - in the sense of the jungle, he's only really far above average at one thing: applying early pressure. And this is because of his generalist-type nature: he's basically on the opposite spectrum of Elise in terms of why he succeeds early. (Mainly because Elise specializes in early pressure with ridiculous base damages, crowd control, and insane dive potential to brute force these ganks, while Lee succeeds off the versatility of his kit which allows him to make pressure happen from many different situations)
Also, his kit is kinda designed to allow him to strongly impact early and dial down slowly as the game goes on from there. (For example, the shield on Safeguard isn't quite too relevant if he's diving to inSec someone as he doesn't get the shield when he hops to a ward, and the effects of Cripple (his 2nd E) aren't as useful because the slow isn't as impactful as it is early game. The two consistencies are his Q execute and his ult - but those are what define Lee Sin.)
My only complaint with Lee is that his R has a stun attached to it - so you can't do anything about it - but it doesn't stun him as well. Other than that, he does have his purpose (all around flexible jungler, strong early and scales decently but not anything to be noted but has decent initiation potential). (Also, I'm saying either make his R stun him (remove R -> Flash) or remove the stun on his R before it hits...)
(Also, the Lethality changes really hit his ability to snowball with damage items, especially since Ghostblade was one of his better items for that.)
At least he has to make a choice in terms of whether he wants to build tank or damage to get one or the other. Unlike certain other champions which build tank items, have strong early games, and maybe build one damage item to 100-0 your entire backline with a spell rotation... I think there are many "specialist" picks that need proper changes before Lee Sin needs something to tune him into a specialist.
(Namely Hecarim, but Courage of the Colossus needs changes too.)