I don't see any balancing problems: the inability to obtain a magical success is counterbalanced by a less powerful, but permanent, effect. It's sort of a middle ground between a useful item and a real magic item.
Given its ability to ignore the Heartless fell ability of the Undead, it would not be out of place if the object also had a Bane of the Undead, despite not being a Famous Weapon or Armour.
It is not our part to master all the tides of the world, but to do what is in us for the succour of those years wherein we are set, uprooting the evil in the fields that we know, so that those who live after may have clean earth to till.