A few years ago I tried it out on FreeBSD, and while it worked, there were some situations that caused problems (like unplugging it when playing audio would sometimes bork the system). I believe that since then there's been a significant rewrite of the uaudio kernel module, so that might work a lot better these days.
I currently use a Behringer UCA202 with Linux daily, and this works very well. I haven't tried this with FreeBSD or OpenBSD, but I can try it for you if you want
