I think the fact that forsaken were rejected for being undead is undermined by the Alliance accepting undead (Death Knights) into the fold yes.
Of course that’s not the only such case. Their embrace of darker powers (Demon Hunters, Void Elves, Dark Iron Dwarves) just kinda cements it further: Like, what, these undead guys are bad ‘because undead’ but the people toying with world corrupting magic and literally tearing the elements asunder are fine? Yeah aight Anduin
They summon a(n unstable) magma giant in Zuldazar in the War Campaign in the goblin genocide quest. Doing so is forbidden by the earthen ring “as the use of such coercion could anger the elements and trigger an event similar to the Cataclysm”. It was a hot topic back when Garrosh was doing it with his dark shaman.
A lot of death knights were mass produced. Especially around the time of WotLK. The Lich King didn’t really raise anyone but the worgen ones, it wasn’t a necromancer of great power raising them either. Looking at Acherus, it was a group of three mediocre necromancers doing it as the prospective death knight they raised had a chance of being raised defective and was therefore put down.
Not to mention that a death knight can and have raised other death knights.
“Warlocks, at the ready!” he shouted. It was always uneasy when those who worked with demons were pressed into service for the good of the Alliance, but they had certain spells—and certain creatures in thrall—whose efficacy was undeniable.
Though Alliance warlocks were pressed into service, it’s described as being an uneasy situation. Nobody involved likes it, but their power is a necessary evil they couldn’t pass up on. Many of the warlocks took sadistic joy in the destruction they caused and everyone’s like “So I’m going to ignore that” because at least it was aimed at the enemy.
The Alliance rejected the Forsaken for being undead. Later, they let the Death Knight’s in because Tirion wrote Varian a letter. It broke the narrative for the Forsaken being outcasts from those they used to stand with because of a stubborn perception that all undead must be destroyed. I don’t think this oxymoron was ever addressed and has become an awkward footnote in the story.
Regarding power, the entire kingdom of Lordaeron and it’s living and unliving population would have been far more useful than a knightly order in a singular floating house.