OpenBSD is unique amongst the BSDs in that its halt(8) and reboot(8) commands use the rc(8) system to shut down services gracefully, so the only thing you're missing is scheduling and shutdown message broadcasts.

Elsewhere those commands simply broadcast SIGTERM/SIGKILL to all processes, so you should use shutdown(8) unless you're in single user mode.

poweroff(8) has halt/reboot semantics on NetBSD, but shutdown semantics on FreeBSD/etc.