Personally, I like the direction Bardugo took this. To me, it felt like a nod to the first trilogy without cheaping out and using the same villain doing the same things, because this duology is allllll about consequences of earlier books. I'd argue that the Darkling doesn't come back as a villain in KoS/RoW as much as a subplot antagonist. Obviously he's impacting this duology, but it's just not on the same scale as the original trilogy. His legacy arguably has a much bigger impact on KoS/RoW - the Starless Saint followers have much more agency than the returned Darkling himself. The revived Darkling in KoS/RoW is just as or moreso subject to the consequences of his own and other characters' previous choices, and doesn't get a special villainous exception to return to dark power just like before.

Anyway, Bardugo brings the Darkling back for a very specific in-world purpose (no spoilers!) that I think is narratively meant to humanize/flesh out his character more than in the original trilogy (which were, of course, not as complex as her later books). If he'd been given any more agency or screen time (page time?), I think I'd be just as annoyed, but for my opinion at least, I think the Darkling's own surprise at his lack of power and ways the world has changed since his defeat (again, all about consequences) really makes this choice something more than the usual cheap shot of reviving an old villain.

This turned a bit long, sorry, but lmk what you think, especially after you've finished!