I wasnt buying it before, now i am not buying it at all. Whole "convenient" aspect of all undead auto-locking to Forsaken by default (aside from a few examples during the first starting quests) was just Blizz wanting to have their pie of "ooooh scary undead raising their victims" and "oh its totes not Scourge and not mind control, they just somehow vicious towards EVERYTHING but their killers".
Forsaken are the source of most dumbass controversies in the game, purely for the sake of "rule of cool" thats not even that cool, just unnecessary edgy and convenient. Like a mallgoth with a cig at the parking lot.
Delaryn and other night elves by their own admission would be better off literally ANYWHERE but the Horde since they will end up fighting their own... but they cant go back to Alliance because they fought their own and feel guilty over it.
That would make a smidgeon of sense if undead were widely prosecuted around Azeroth, but like every second organization, collective, group, coven, clique and band accepts the undead members no problems, even if they also have living, Alliance or fucken Light elements.