Y'know.. if Fiora is dealing 2000 true damage in one auto attack, she's going full glass cannon against a full-blown tank. That means she's incredibly squishy, and her main value is in her ability to assassinate. That means teamfights will be the bane of her existence with all the CC flying out. Also, ADCs aren't countering a tank's entire build with one item or tanks wouldn't largely be dominating the meta right now.. Last Whisper was hit with a pretty enormous nerf, y'know.
Also, a lot of those complaints sound less like power where they shouldn't have it, and more like power that is explicitly given to them for them to use. They're just using what Riot told them to use, not even exploits or anything like that.
Irelia is a weird case and everyone knows it, I won't comment on that one.
Fiora was meant to be able to duel literally anyone, and preferably make it quick. That's how they accomplished it - she fights everyone equally. If she insta-gibs your tank, then your tank already did his job by very likely soaking her ult, and most likely CC'd her at least once. If you couldn't follow up on shattering the glass cannon Fiora, that's on you...
Juggernauts are durable, but they are not more durable than tanks. You are not competing with scaling health shields, armor/MR steroids, etc. when you have none of those. They do more damage, because they were designed to, but they are not at the tanky level that a real tank is. Darius will never be as tanky as Sion, Malphite, Rammus, etc., nor will Skarner, or Morde, or Illaoi. Garen might, but that's only while his damage reduction is up.
ADCs I already covered.
Ekko's main damage is in his passive and the second hit of his Q, both of which can be avoided throughout various methods in the game.
LeBlanc scales hard and ends the game fast, or falls off because her highest burst combo is relegated to a single target with one spell doing AoE damage at a time when the game demands teamfight strength.
Lee Sin is a case I won't touch, because I honestly tend not to care as much as other people. He could use some tuning down but he really doesn't need as much as people say he does.
Tahm Kench is not a tanky assassin. He's stupid tanky for the damage he does, but damage in his fashion does not an assassin make. His primary problem comes with his W being a lose/lose for any enemy he's near. Either he eats his teammate that's in trouble and your only option for attack is this super-tanky champion with 2 effective health bars or he eats you because his teammates aren't in danger and you lose significant amount of health. The main problem with Kench is that you can't be in a favorable position against him unless he's alone.
Frost Queen's is funny.
Tristana was stated to be a tower-destroying ADC, the premier siege marksman - the whole reason she can bomb the tower is to enforce that.
There are a lot of champions with power they don't need (why does Lee Sin have true sight on his E? Things of that nature), but the examples present aren't as superfluous as actual cases. LeBlanc has scary scaling numbers, but it's less intimidating when you look up from the theorycrafting math and see the practical application of that math. Juggernauts are durable, but they kinda have to be when they have to be in the middle of the enemy team, with often very little way to force themselves there beyond 'run at them where they can see you,' and live long enough to do damage.
Tahm Kench is... Tahm Kench is stupid.
Just remember, you don't have to agree with me or anything, this is just the way I see these things.