This is because of base dmgs and low cooldowns serving as dps in the case of ekko akali and fiora.
Yi shouldnt be as much of a problem now if the nerfs were of the correct degree. See, they nerfed the base dmg on his e and now he cant abuse it with a tank build + devorer.
For a while, its seemed like a pretty solid dynamic in this game that ppl that were hard to kill werent very threatening in and of themselves, because they tend to lack significant dmg and they tend to take a long time to aquire a target due to lack of mobility and range. They do however, posses cc which protectes them, holds an aquired target in place long enough do get into a position to deal dmg, and also enable a nearby ally to deal dmg more safely than otherwise. On the other end, ppl who were threatening in the dmg they dealt tended to lack defense, and were easy to kill.
However, with this season we have many defensive items that "double dip" into offensive stats as well. Some even triple dip into defense, dmg, and cc. And that breaks the aforementioned dynamic. The ones building these items are hard to kill and yet deal quite enough dmg, if they are given the time and position to acquire an enemy target and deal their dmg.
You should notice that the champions you listed are all champions with high target accessibility. Fiora has high base ms and a low cooldown dash, and a slow. Ekko has range, a dash on a moderate to low cooldown and sticking power through his slows. Akali has three dashes in a row, potentially. These are examples of innate target access. Their abilities give them the means to acquire a target quickly and easily. This is what facilitates their ability to build these double dipping items. However, it is abusive because the defensive stats of the items are tuned to champions who take a long time to acquire a target, but the guys we are talking about acquire targets quickly and easily.
Now, the dmg stats of these items are probably not that abusive. Its really not a problem that akali has the dmg to kill you with this build, and in fact she can only kill you rather slowly. The problem is that once she has a target, neither you nor anyone on your team can possibly kill her before she kills her target (because of the defense stats) and because she has high target access from her kit, you cant run from her in any way.
So now, paradoxically highly offensive kits synergize with highly defensive items. No one really thinks this is a good idea.
Also, there is sated devourer and this item is blatantly overpowered. This one item is worth 3 item slots. Any champion that has a reason to build it over a standard champion should build it. Standard champions should build it too, if time permits.
Case in point: I was loosing a game to a sated yi. Then he lost his peanuts and sold devourer for a bloodthirster or something. Afterwards, we easily killed him and won the game.