Right but, why bother with smoke and mirrors when you have an immortal unbeatable army in the first place.
Even in the book its not really explained why the oathbreakers only help out for one fight; the only difference is in the books they never actually go to Minas Tirith they just help against the corsairs.
It would be more like, they invented the atomic bomb, use it to take Iwo Jima and then the United States goes back to sulking and wondering how the hell they’re going to take the home islands.
In lotr they use the undead to win the battle of pellenor fields but they don’t use it to win the war. It’s a legitimate plot hole.