I'll give you the one on Pyke. I think the idea of an "Assassin/Support" was counter intuitive to one another was a rather poorly thought idea. After all, what are the two classes intended to do?
Assassin: Eliminate high-priority targets.
Support (Whether Enchanters, Tanks or otherwise): Keep people (Mainly the ADC) alive.
The descriptions above might be very generalized, yes, but the two ideas, when put into a single champion design, kind of contradict one another I think.
I personally think it was another one of Riot's attempts to try and make Support more attractive to play, or provide those that get Auto-filled someone that may resemble closer the style of champions they play (If they play an Assassin, Skirmisher or so on).
Darius's (unlike Pyke's) Ult/Execute is not GUARANTEED to kill the target under a certain threshold. Before you say anything, what I MEAN by that is for Pyke, as long as the target is within the threshold indicated, it doesn't matter how much health they do or don't have; they're gonna die. Pyke's execute, unlike Darius's, kills THROUGH SHIELDS, while Darius's does NOT. If the target has a shield on them when Darius lands his ult, that amount of Pseudo-HP is taken into the damage calculation and through it, they may survive that main blow. They may not survive the bleed that follows, but if they survived the MAIN STRIKE, they have just forced Darius's Ult on cool-down, greatly reducing his threat-level. There HAVE been cases, when I've played Darius, where someone that survived the main strike but died from the bleed afterwards, where I was then granted the opportunity to reset. I don't have any footage or video of these occurrences because they have been so few and far-between. So much, in fact that I'm almost thinking those times that it DID happen were because of a Bug/Glitch.
Darius's True Damage Execute has much more counterplay to it than Pyke's. You mention it yourself with the fact that he has to apply all 5 stacks of Hemorrhage to get the maximum effectiveness, and yes, this is more difficult for immobile characters than more mobile ones. In addition to that, there is the well-timed heal or shield (Or other effects, such as an Invulnerability like Kayle's or Taric's, or a form of untargetability, but that's another subject) that I bring up now, that can allow the target to survive the main blow, thus forcing his on CD, while Pyke, so long as the target is within the threshold; doesn't matter if they have a shield on them, unless they get their ACTUAL HP (Not have Pseudo-HP ala a Shield) above that threshold before he strikes, THEY WILL DIE.
I have my own problems with Fiora's True Damage, but hers has counterplay as well like Darius (I'm NOT saying the methods of counterplay is exactly the same between the two, I'm just saying that it HAS counterplay). That counterplay is positioning yourself to where she cannot reach or strike the current showing Vital. That may mean you have to go back and forth out of the activation range until the Vital is placed in a more advantageous position, or placing yourself against a Wall in such a way so that she cannot strike it. Considering she has that small-scale Dash that prioritizes the Vital as the target, it's easier said than done, but it's NOT impossible either.
The Problem with True Damage nowadays is that it's MUCH TOO PLENTIFUL. It used to be a more niche aspect, in certain kits but not available to everyone. The most obvious provider of True Damage nowadays is the Conqueror Rune. Granted, it's only a small amount of True Damage, but it is nonetheless there. Because it is on a Rune, it is available to everyone in a literal sense, because a player can CHOOSE to take the Rune on their chosen champion, even if said Rune doesn't have much synergism with that Champion's Kit; they can still CHOOSE to take it, and thus it is available to all.
A scaling, true damage, resetable, aoe, execute. Let that sink in. Yea, Riot was massively r%%%%%ed on that one.
Yet another true damage, resetable, scaling execute. Luckily it isn't AoE. Yes, I know he has to stack it but does that really matter when you're playing an immobile melee champion? No. That's why Darius is such a strong pick. I have never seen a meta where he wasn't strong. True damage never falls off especially on an execute that scales and resets.
%max hp true damage that heals herself. Why couldn't this just be bonus damage or just plain true damage like old Irelia had? Why did it have to be % MAX HP true damage? This is just stupid
and 
(unpopular opinion) are the only two who use true damage and/or executes perfectly fine. If you're the villain then let your team handle him or kite him. For Ahri just dodge the 2nd part of her Q (I'm honestly not even sure if she has true dmage on her Q anymore I dont play her ever)
It is much much MUCH more understandable if a melee champion like Fiora is dealing true damage.
But a ranged stealth champion dealing true damage is too much.
