Another thing that comes to mind is wtf Greed was thinking.

Unless he means he's got a limited lifespan, that body beats a transfixed soul anyday.

We've already seen the fundamental flaw in walking in empty armour.

Even if it wasn't controllable, Scar's technique, as well as other Alchemy techniques can just smash it down. Since Alchemy is about destroying and rebuilding matter, anybody with an ability to alter metal would kill you.

If you can't have indestructibility, the next best thing you can hope for is regeneration.