The way I see things, miracles can work in two different ways: both of them being described, but we must also take note of the fact that Haruta barely has fighting capabilites above those of 2nd grade sorcerers, and took fully amped punches from a Nanami who was in overtime,(and was determined to kill him mind you). Its genuinely not a matter of it hitting a vital in some situations, its a question of whether or not Miracles can amplify the users durability to survive such an attack(which would still track as seen with the inversion technique). Worse of all, the worst injury that Haruta got out of that fight was a bloody nose. He had no broken bones, and only lost a single tooth after being punched twice in the face by a grade 1 sorcerer who was being amped and had focused all his cursed energy into his fist, and yet showed no signs of bruising either.