Aye, you cant cast armor on it. Still, the question stands: how do you remove it from the field? How do you stop it? As QT said, its almost like an immortality, and for such status, damage/cost is high.
The difference between this and immortality is - owner cant target immortal creatures, this one he can, but obviously wont. Opponent cant target immortal creatures, this one he can, but most likely wont, as no one wants to waste 5 CC cards to kill a single creature. Mass CC maybe, but 5 mass CC cards are super rare sight in one deck.
Am I missing something?