Top:
A lot of people think highly mobile champions are hard to master. But try to play correctly these champions.
Shen has a high impact ult, and strong W, but those can easily make or break fights depending on when you use them.
Gnar has a particular mechanic that makes him really hard to master. Many focus on his mini form for high damage, but his mega form is the one you should focus on, and you have to be pretty good in order to initiate a fight at the right moment. He's sort of a tank that's susceptible to bursts (in mini form)
Mid:
Azir is supposed to be hard to master, in many ways he's like an adc without sustain, which says a lot.
Karma has just a weird kit, half burst mage half support. Similar to orianna, but the latter has clearer ways to interact with opponents and allies alike.
Syndra and Viktor have lower ranges than your average mage, without having reliable ways to soak up damage (viktor'q shield is pretty laughable). They have high bursts than can easily delete squishies, but have very low target selections, meaning that they'll hardly reach and adc without help. Theyr bursts, while high, can't really deal with tanks, so a proper frontline can easily deal with them, and may force them to waste their CDs on low priority targets. They're also weak to assassins, like all mages are, especially viktor, whose stun takes a while to work.
Jungle:
Kindred's new stacking mechanic didn't really solve her thematic problem, and now the scuttle changes forces her to fight early, when she actually doesn't want to. It's not just that the meta doesn't favor her, you NEED, more than many other junglers, to counterjungle as often as you can, while constantly looking for possibile champion kills. Otherwise you get no stacks and become irrelevant pretty fast.
Gragas is neither a tank nor a damage dealer. He can't sustain as much damage as your average tank, despite his W and passive. You can build damage on him, but you'll be able to achieve takedowns only if you manage to connect all your spells together, usually hitting W and pushing a target with your E into a charged Q. He has disruptions, but struggles to pin down targets, and can't really initiate teamfights, unless he engages not from the front, but from the side.
Not much needed to about lee sin, everyone nows that he's fun but very hard to learn. Still, even you struggle with the ward jumps and the Q-R combs, he can be a decent off-tank.
Adc:
They are more difficult mainly because they need a proper team more than other adcs.
Kalista painfully needs a decent support (that is, a decent player), and her opping mechanic, as it is right now, is hard to learn properly while still not saving her from all-ins.
Lucian has one of the lowest ranges among adcs, but unlike
, he has no way of dealing damage to the backline. He can't win attrition fights, he needs to constantly dash around while being relatively close to the enemy.
Tristana right now starts with only 525 range, and by lvl 18 barely surpasses caitlyn. Also, she has no skillshots with with to poke the enemy, or even secure some cs from a distance. She can only deal damage through her autos, even her E is more of a AA amp than an actually spell. And her AA isn't insane to being with, she's no
or
. She has good self peel, but you need to use it wisely otherwise it's like having nothing.
Support:
Bard needs to roam in order to stay relevant, even as a support, but that doesn't mean that you can leave your adc anytime you want. You have to learn to balance laning and roaming effectively. Sure, if you get a lot of chimes you can deal some serious damage, but it trying to get there can easily lose you games. Also his kit forces you to think, a lot.
Thresh has a bit of everything but not enough to be unbearable. He's one of, if not the best champion, at least in terms of kit. He can peel, shield, damage (a little), even engage. He's a jack of all trades, and as long as you can balance all his abilities, then he's a beast, otherwise you'll have many dreadful games.
Pyke has just been released sure, so people need some time in order to learn. But what i want to say right now, is that he's a half support/half assassin, which means that, despite wanting to build a lot of AD, his kit doesn't really allow him to kill targets on his own. No matter how much damage you think you can do, you always need at least on ally in order to win a fight. His ult might be the best clean up skill in the game, but you can clean anything if the enemy is still at high health, and your best weapon at that moment is cc, not damage.
With all that said, in pretty much all metas I say that the hardest champions to play are the ones who need good team coordination. That doesn't mean that their kits have to interact in some way with your allies. It means that in order to be effective, to succeed at your role, you really need the cooperation of your team. Viktor doesn't have interactions with his teammates (aside from a bit of cc), but you can never fight on your own, you're just a moving target in that situation.