As really PFsense seems to just use a single main filesystem anyway, I just loaded it up with UFS instead of ZFS, as drive redundancy is handled by the RAID controller itself. Just funny that UFS will just boot and run, but selecting ZFS ends up with a non-working system. I have another almost identical server, but it's running Ubuntu, but with a ZFS root, and it boots just fine, so a FBSD quirk for sure..