People think it will make the army they lead invincible.
The person who wields it could rule the world.
Plot device category.
Total GM fiat.
Our GM postualted some rediculous rating of like -30 refresh (pretty sure it was just a way of saying no one can use it, it uses them---it's powers are in theory immeasureable.) Removing the free will of the critter/person holding it.
Seen plenty of stories where the spear isn't holy, but unholy.
Also the comic book Magdalena has a tamer theory on it. The descendents of Mary Magdalene use it to hunt demons for the Church. (those are very broad strokes, but basically it kills monsters.) I think a female order like a cross between Slayers from Buffyverse and Knights of the Cross from Dresdenverse would be appropriate. I really like that idea.