With Unbreakable Vows, the... arbitrator?... sacrifices a portion of their magic permanently yes? One issue is that, after you die you might need that magic for something, like the more magic you have the more pleasant (or less!) magically created heaven is. In any case, even if magical society was fine with sacrifices, they might reason thus, and not use unbreakable vows. Such a society would make investigation (magical!) into potential afterlife a top priority, so lack of use of such a ritual might be compensated by finding out there is a heaven (or hell).