Great question — answering as the UphillTech team:
UFS is less resource intensive, ZFS is more resource intensive, usually needing a bit more RAM, so it's not a great fit for VMs. Beyond that, at least for me, ZFS is preferred. One of the reasons the installer doesn't offer it as an option is simple that no one has made the changes to the installer to make it do so. But, there are folks working on this. The changes may even make it into the 10.0 release. I can get you more details on this if you'd like to test. Or, if you're just looking to get a system setup with ZFS root, you may want to look at mfsBSD .
Full answer by Steve Wills: https://superuser.stackexchange.com/a/662535 (CC BY-SA 4.0).
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