Which faction poses the greatest challenge for the Dwarves?

I disagree with you. But I'm writing this not for you and not to argue with you (I don't see the point in that), but for those who don't consider my advice useless.

Soldier's Luck increases the chances of stunning by vampire princes (they, like lords, often attack without receiving damage in return, and the princes' stats are higher) and the chance of stunning by skeleton warriors. Skeleton warriors are, of course, quickly destroyed (in your hands, zombie peasant), but you don't have to expose them to attack. And even if there are few of them left, they can significantly help with their stunning.
The fact that Luck is taken as a skill will allow you to put some other useful artifact on the luck slot instead of an artifact that increases luck. So the argument about losing a slot is simply absurd. Luck will not be superfluous for any race.