I think like many formalisation, they are a tool that is useful within the right culture, but are useless in the absence of that culture. The Rust Community is an excellent example of a community with an effective and well-enforced CoC that has contributed towards creating a community that is renowned for being welcoming. But I suspect that has at least as much the attitude of early community leaders (Graydon Hoare, Brian Anderson, Huon Wilson, Aaron Turon, etc) on these issues and ongoing excellent moderation of community platforms from a dedicated team of moderators.