Comment by Dagger2

They weren't glossed over. Dual-homing is handled in RFC 3484 (https://datatracker.ietf.org/doc/html/rfc3484#section-5 under rule 5) and ND is designed to detect unreachable routers and stop using them (see e.g. https://datatracker.ietf.org/doc/html/rfc1970#section-6.3.6). You can also specify router preference in RAs (https://datatracker.ietf.org/doc/html/rfc4191).

They thought about these things and put features into the protocol to support them. This is, once again, the opposite of your belief that they didn't.