I vaguely recall some interview with or blog post / talk by an OpenBSD developer, where they said that POSIX tended towards adopting whatever GNU/Linux did, and that the BSD crowd's interests were not taken into consideration that much.
I don't know much about the process behind POSIX/SUS, but I can understand how OpenBSD developers wouldn't be super enthusiastic about POSIX compliance if they felt their input was falling on deaf ears.
FWIW, I did use the SUS as my main reference when writing a few hobby projects, and OpenBSD gave me no problems whatsoever. macOS, on the other hand, which is a certified Unix, did not support barriers at the time (that was ~10 years ago, I have no idea if Apple added support since). (I know barriers are optional, but come on.)
I don't know much about the process behind POSIX/SUS, but I can understand how OpenBSD developers wouldn't be super enthusiastic about POSIX compliance if they felt their input was falling on deaf ears.
FWIW, I did use the SUS as my main reference when writing a few hobby projects, and OpenBSD gave me no problems whatsoever. macOS, on the other hand, which is a certified Unix, did not support barriers at the time (that was ~10 years ago, I have no idea if Apple added support since). (I know barriers are optional, but come on.)