Wayland support on BSDs and other non-Linux platforms has lagged behind, but Johannes Lundberg is one of the FreeBSD contributors working on getting things into shape. Besides Wayland/Weston support itself, it's also a matter of the DRM drivers being ported from the Linux kernel being in less than ideal shape generally compared to the upstream kernel code.
As of this week, Wayland 1.12 is working on FreeBSD as is Weston 1.12 while more porting is still in-progress. XWayland is working too when started manually, the Sway i3-compatible compositor is also working, GTK3 applications on Wayland are working, and it appears the SDL2 Wayland back-end even works too. But GNOME's GDM with Wayland isn't yet working due to a dependency on logind, and some other issues still to address.
FreeBSD users wishing to play with Wayland/Weston can find out more information on the configuration approach and other details via this mailing list post.