Ubuntu has only fraction of what FreeBSD has when it comes to ZFS.
Several things that Ubuntu misses:
- lack of ZFS Boot Environments - https://is.gd/BECTL - https://is.gd/BEADM
- no bootloader integration
- still requires ancient EXT4 or XFS /boot to start system
... and while the version of ZFS is newer on FreeBSD you can always install these packages and have LATEST OpenZFS available:
- sysutils/openzfs
- sysutils/openzfs-kmod
Hope that helps.