The only downside is several of the *BSDs don't have wayland. Not all the world is linux and sometimes that is a good thing to encourage.
You wouldn't pick BSDs for a desktop anyway, assuming you want a working system and don't want to use Windows/macOS.
Now the concept of a "working system" can be a bit hard to grasp for some people, so let me put it this way: There are vastly more things that normal people do on their computer that work on Linux than on BSDs.
at least FreeBSD does already have Wayland. It has a few more hiccups and rough edges than on Linux (at least from my PoV on running it on an older laptop) but otherwise it works fine.
Here is a short setup guide on how to get it working rather nicely: https://thesaigoneer.bearblog.dev/freebsdkdeplasma6wayland/
There's nothing about Wayland that ties it to Linux. Wayland compositors tend to require a few features exposed by the graphics drivers (e.g. DRM) but there's nothing stopping the BSDS adding these (iirc FreeBSD already does).