Shaco's entire game is related to his early game. If you don't do well early game and get far behind, you're going to be doing nothing but split pushing more than a Tryndamere for the rest of the game.
A fed Shaco can do really well and be scary to the enemy team, but it's difficult to get fed early game with him due to his low damage output. He doesn't have much attack speed, he's melee and has no CC besides a meh slow, plus he's very squishy so a lot of time I see a Shaco gank there's a large chance he dies.
I'm not saying his early game is bad, but his ganks aren't the best and require someone to CC them or just make sure they follow up 100% perfectly or it can VERY easily result in you just feeding their lane.
His jungle control is really nice though. His clearing sucks though without his boxes but due to his Q can manage to counterjungle and steal camps quite easily. It's kind of awkward, he has almost no clear time without his boxes while also taking a lot of his mana up, but he has amazing control over his jungle and the enemy's jungle at the same time.
Shaco is just a fan favorite like Teemo, he's old and outdated and not really viable unless you main him and are a perfect Shaco. The reason why he may never be given a rework is just due to him being a fan favorite. I'm not saying you can't Shaco work, but there's people who can clear, gank, and assassinate much faster with more sustain, damage, and better early, mid, and late game than Shaco.