Very spot on, Tymell. This was the first time the WE worked for me. I didn't even like them in Betrayer. In here, they come off as very much more realistic, having this super-abusive relationship with their father, that is so much based in real world dysfunctional relationships. The kids craving for attention, going to whatever self-destructive lengths to impress, while he just doesn't care. Only some of their siblings go the other route: If he doesn't love me, despises me even, maybe he isn't worthy of my adolation at all - and so they rebell and sitance themselves.
This works even better, considering that the lore sometimes likes to point out how stunted Astartes can be, because they are taken in as children and put through hell. That's why they have the close sibling bonds but tend to look to parental figures. And that doesn't always work, because some parents actually suck. The WE being masters of aggressive warfare and at the same time being somewhat neglected children in their mental state made them a whole lot more endearing to me.
If we now also consider that the WE might have inherited that basic empathy trait of Angron, they would even more care about satisfying their father, about making him proud. And he abuses it without a second thought, actually tries to hurt them as much as possible, as he was hurt before on Nuceria. From a psychological viewpoint, this story made them fascinating.
Also, I really felt for Mago - if a WE stays loyal, they tend to show that strange, but impressive sense of martial pride and honour that gets me every time. I would go so far as to say that the story would have been even better, had it been a full-length heresy novel. Nevertheless it worked quite beautifully for me.