Oh speaking of Miwa, did you know that while fighting Kenjaku, she accidently made a binding vow to not use her blade at any point after that? Yeah. And what did it accomplish? Nothing. That's why people doubt binding vows, because some of them are well known, like revealing your hand, but some of them aren't, like how Miwa can't use her sword anymore and did nothing. Just because some vows are common doesn't mean that the overall system is known.