Ehh...I agree that feats would have been the better solution (I still allow the WgtE feats in my Eberron game, in addition to the dragonmarked races). I don't think I would agree it's in contradiction to Eberron lore, as Keith has always been pretty clear that classes are a metagame tool that you should be using to build your concept, but don't have an actual presence in the narrative. Just because the fighter class isn't good with dragonmarks doesn't mean that the concept of warrior with no magical training has a powerful dragonmark isn't realizable; you simply use a different class framework to do so.
I get where you're coming from, and it definitely true that Keith Baker has always been a massive fan of reflavouring, but it is really difficult to convincingly build a "warrior with no magical training but a powerful Dragonmark" under the structure they've given us - and that's not exactly an unusual concept for Eberron either. Whereas with the previous, Feat-based system, it was trivial to build it.
Another thing to note at this point in time is that we all have to remember that this UA is not the time to complain about math balance with any of the subclasses. As is always the case with these articles, WotC purposefully does not worry about the math when they present there things to us.
I get what you're saying, but history suggests that if a class looks broadly underpowered or overpowered, we should say something now, because WotC have had some problems there, historically, and there is unlikely to be another UA version of these unless the Psi-die mechanic is rejected (and if it is, I suspect we're looking at years before we see another version of them, unless they're super gung-ho for Athas, like have actually started writing it).
I'm not worried about the precise numbers but I think it's absolutely fair and the sort of feedback they want to say stuff like "this feels weak" or "these numbers seem" low, or "the odds of losing your dice entirely seem way too high". Stuff like needlessly using a d4 for the second Psi-dagger is also just weird and doesn't help. It's only -1 damage, but it's like, why add that complexity?
Heh... it's a game mechanic. Do any game mechanics "feel" like the thing they are supposed to represent? Does adding +2d6 of damage "feel" like actually sneak attacking someone? Like I've never gotten the "feel" that any god is granting me further insight when the cleric gives me a +1d4 for Guidance. It feels like the cleric player just telling me "hey, roll an extra die!"
Is there a particular type of mechanical expression they can create that "feels" specifically like psionic power? Has there ever been?
Yes to both your questions. Game mechanics can have a feel (for most people, maybe not for you). Sneak attack dice absolutely do feel like making a deadly sneak attack to a lot of people. They might not to you, but I can't help you there.
And the whole design of the 2E Psionics system felt very "Psionics"-y, and not just because it was the first such system I met (it wasn't - I'd seen others in other games before that), but the whole structure and flow it had worked really well for making it feel that way (even if it was as clumsy as most 2e systems). Far better than some contemporary systems which just felt like random superhero abilities, rather than a cohesive whole.
That doesn't mean we need to replicate it of course. Or have anything similar.
But the "random increases and decreases in power over which you have no control" has nothing thematically to do with conventional and D&D presentations of Psionics or similar abilities. Indeed, Psi is traditionally, in both D&D and fiction, typically about control. If you had dice to spend, that would make more sense than this. I get that they already have a couple of mechanics like that (actually several), but that's more "Psionic" than random increases/decreases in power. It's the random increase/decrease element which doesn't fit here.
That's not to say it wouldn't be a good mechanic for something, but Psionics? Not really. And the sad thing is, I think if people say that, they're just not going to put these subclasses in at all.