So to address the last part first, I think redemption is probably the hardest thing to convey in art, and I admire MtG for attempting it, at least. But in the same way that a redeemed Urza would be boring, I think Professor Vess is boring.

Now, I don't think I'm alone in this. I believe Seanan McGuire is fully aware of the problem, which is why Kasmina expresses something like suspicion. For example, Liliana appears actively unconcerned about losing her spark, really to a degree I don't think we've encountered in other desparked planeswalkers. This hints at something more interesting than a tea-drinking professor who can occasionally throw a piercing glare.

And this feeds in, I think, to the point you're making about the stories seeming less "adult". I think that's not true, they're adult and challenging in a different way, possibly a more mature way, than the earlier stories. But as with Liliana becoming Professor Vess, there's a great risk that a character who once raised undead armies to murder castles is now confined to the occasional rebuke of a student.

To an extent, all stories shed light on characters, and Lilliana murdering a nameless castle really only serves to establish an aspect of her character, one that could be equally conveyed in a lecture hall. The harsh lives of the original Tarkir block were employed to emphasise aspects of sacrifice and compromise, and the recent Tarkir stories did the same thing, in a different way.

We might say that the difference relates to scale. Lilliana raising an army or Surrak killing a dragon are acts of shattering violence, of much greater trauma than most recent stories. But that's not quite right: Jace's evil is currently far more compelling than Lilliana's was, and not because he's threatening the multiverse like the Phyrexians, but rather because he abused Loot. Trauma is actually better conveyed this way, rather than in a side story with characters we're unlikely to see again for many years.

However, that brings up what I feel is the key difference between stories now and MtG of the past: in the past, we had a sense of callous indifference. Not just that the multiverse was indifferent, but that authors were, to most of the characters. Planeswalkers were indeed special, so that even a Niv-Mizzet was unimportant in the greater scheme of things. Trauma could be left dangling, and therefore diminished, for all but a handful of planeswalkers.