This story, like many others, goes to show just how powerful Arthas was. In addition to that, I've come to realize that Greymane more than repaid the favour the Alliance made him during the cataclysm, considering his actions in Stormheim saved everyone from having to deal with armies of rebuilt and zealous undead. Betrayal would surely aboud within their ranks if entire armies were rejuvenated as Nathanos was because it is his living experience of Sylvanas that keeps his loyalties in check, one which no other undead has.