Shaco problemos (Wall of text)
Gonna go ahead and soapbox here about how Shaco needs looking at. Feel free to disregard.
The meta has gotten progressively worse and worse for him since he was nerfed a couple of years ago explicitly because he was 'being too oppressive in lower rating'. Consequently right now he's very hard to do well in higher rating with. I'm not saying you can't do well with him, it's just much easier to pick something fairly mindless like Gragas jungle and run around the map bonking people with your barrel if you want to do well than it is to pick Shaco and risk it all on being able to make something happen early on.
The changes to dragon and dragon timers mean that it is no longer possible to sneak dragons past the first one and that there is practically no value in any dragon past the first one in a game with Shaco in it in solo queue; neither team is going to get five dragons unless it's an ridiculous stomp. I imagine Riot won't be changing the dragon mechanic until the next season of course, I'm just pointing out that it was a particularly nasty change for Shaco. Before if Shaco couldn't gank any lanes because they were bad matchups for him he could just take dragon control and win the game with that advantage, now he can't possibly win by taking dragons.
The introduction of other damage junglers like Rengar and Khazix have also very much displaced Shaco in his role. As soon as Rengar hits level 6 his ganks are much, much stronger than Shacos, with his ability to oneshot an ADC and then afterwards still have a handy movementspeed, armour and magic resist boost that lets him followup or disengage. Now, Shaco has levels 2-5 where he is more impactful, but as it turns out it's not a fair trade-off. You certainly don't see Shaco being banned in high rating play over Rengar. In high rating play Shaco tends to play more like Lee Sin now, with the early game advantage but limited usefulness later, except that if Shaco falls behind he can't possibly build tank and have any impact whatsoever in the game like Lee Sin can with his dragon's rage catch-potential. Being exceptionally weak past level 5 doesn't really make up for getting a couple of decent strength ganks in early on, which can be replicated by other champions like Reksai in any event.
It tends to be the case that Shaco can't 1v1 anyone as an assassin unless he is ridiculously far ahead or they are a specific small group of champions that have no defence against someone being on top of them such as Ashe. I mean, he can also beat Zed or Lee Sin if he makes some very good plays anticipating their skills and dodging them, but it's pretty goddamn hard to do that frankly and I don't think there are any other champions he can do that against. Past level 2/3 cheese he tends not to be able to 1v1 any other junglers in the game, especially any of the top pick junglers right now, which means that in some games he just gets shut out, and since Shaco can't farm for crap he falls further and further behind and counts on his team to 4v5.
Incidentally, all the higher level (AD) Shaco players all rush tiamat right now without finishing the jungle item. That seems like it shouldn't be the case. Needing the item that much.
Reading the patch notes now, apparently red buff is much less useful for Shaco now since its slow is out-valued by Shaco's crummy 10-30% slow also.
Honestly I don't really know what would make Shaco better balanced. If you gave him something ridiculous like a reset on kill/assist on his Q he'd be way too strong, so I suppose all that could happen is buffing his numbers a bit or reverting his nerfs. Maybe his farming could be buffed somehow so even if he gets wrecked early on he can still farm without falling even further behind. I seem to recall quite some time ago there was a rework planned and his E would no longer slow and his ultimate would do something 'amazing' or something but I guess that was scrapped and he's been completely forgotten.
Last note is that Shaco's Q is bugged. If you smartcast deceive while in auto range of someone then it will drop stealth when you appear at the place you deceived to. Pretty sure it starts up an auto that drops your stealth on the thing you were in auto range of, but that auto is cancelled so all that happens is your stealth is gone which can get you killed quite often.
tl;dr: Mad because bad. Also Shaco hasn't been changed in years and the meta has been getting worse and worse for him.