My conception of the Druchii is that, in some ways, they actually are famous for their discipline.
The Druchii are defined by their ambition and arrogance, viewing the entire world as merely a buffet on which to gorge themselves at their pleasure. But they are also defined by their hatred for you, the Asur, their yearning to wreak vengeance upon you, retake their ancestral home, and assert the primacy of rule by conquest over all 'lesser' justifications - and this unifies them. It is a central, foundational mythology of Naggaroth as a nation and the Druchii as a people - "we are the true elves, unjustly forced from our rightful homeland by the craven schemes of our effete, degenerate cousins, who we will overthrow and dominate in the name of Dread Malekith."
Culturally, Druchii ambition and infighting is governed by a sort of swinging pendulum, a sense of time and place. Vassals are allowed, even encouraged, to reach for more, more, more, even at each other's expense - so long as doing so doesn't sabotage larger concerns. If a group of Corsairs are sacking a town, they all expect each other to pounce on any chance for loot and spoils that they can clearly claim (or pocket on the sly) as solely their own instead of something collectively taken to be divvied up by the captain - and in turn, they all expect each other to come running if one of them encounters resistance serious enough to warrant calling for aid. Even if you have ongoing fueds with them, you still drop everything and help, because whatever you are out there, here in the fight you are comrades, closer than family, sworn to each other by the sanctity of bloodshed under the molten iron gaze of Kaela Mensha Khaine.
Warfare is the great unifier of the Druchii, a way for them to focus their aggression and hatred at an outside foe, and prove the military acumen on which they pride themselves. "We aren't like those oafish greenskins, plebeian humans or amateurish Chaos-worshippers," say the dark elves, "they think war is just about being good in a fight. We know better. We take this seriously." These are elves, after all; sophistication and artistry is prized, though it be cruel and murderous. So to undermine a campaign or a battle with personal scheming would compromise the Druchii's self-image as the supreme masters of warfare, which necessitates a more disciplined and professional outlook. This is, of course, never more true than in wars against the Asur, when the opportunity arises to visit racial vengeance upon their hated kin. For all of these reasons, while the Druchii are traditionally ruled by self-indulgence and internal strife, open war sublimates these desires into a chilling focus and tightly-governed discipline.
