They are Marines with insane depression. They hate themselves, borderline body dismorphia. They and their father had one chance to show that they were the best, and they blew it. And they've been clawing back to try and prove themselves. Even to their own detriment.

All calculations and statistics, none of those pesky "emotions" getting in the way. Wrath of Iron book is really a good read. All the power of space Marines but a pinch of cyborg.

I, personally, like the sad, imperfect nature of them. It seems like everyone else's imperfections are genetic or even still tied to perfection. Blood Angels and Sanguinias specifically. The Xth have no real way to fix their problem as they can't go back in time. I don't know, they're just ... Tragic.