You should install Freenas onto virtualbox and create 3 virtual disks to see what options UFS supports. I had a UFS mirror for a while that worked fine. Also according to one of the devs on the Freenas forums UFS is being completely removed in the future. Freenas definitely looses its allure for me when you have to use hardware that supports ECC ram and massive amounts of it for your custom NAS build.
My experience is that after I heard about UFS support being removed I went looking for other NAS software and found OpenMediaVault which is Debian based and amazing in my opinion. It doesn't have the Jails feature of Freenas but you can accomplish all the same things you can do on Freenas.
I have an OpenMediaVault NAS running right now with Plex and Transmission installed, I can also run a Minecraft server on it and Owncloud. Look up omv-extras if you do decide to use it.