I tested again with xenial-proposed and confirmed that everything looks good as far as systemd no longer failing ZFS units due to no /etc/mtab.
ii libzfs2linux 0.6.5.6-0ubuntu14 amd64 Native OpenZFS filesystem library for Linux
ii zfs-dkms 0.6.5.6-0ubuntu14 amd64 Native OpenZFS filesystem kernel modules for Linux
ii zfs-doc 0.6.5.6-0ubuntu14 all Native OpenZFS filesystem documentation and examples.
ii zfs-initramfs 0.6.5.6-0ubuntu14 all Native OpenZFS root filesystem capabilities for Linux
ii zfs-zed 0.6.5.6-0ubuntu14 amd64 OpenZFS Event Daemon (zed)
ii zfsutils-linux 0.6.5.6-0ubuntu14 amd64 Native OpenZFS management utilities for Linux
I tested again with xenial-proposed and confirmed that everything looks good as far as systemd no longer failing ZFS units due to no /etc/mtab.
ii libzfs2linux 0.6.5.6-0ubuntu14 amd64 Native OpenZFS filesystem library for Linux
ii zfs-dkms 0.6.5.6-0ubuntu14 amd64 Native OpenZFS filesystem kernel modules for Linux
ii zfs-doc 0.6.5.6-0ubuntu14 all Native OpenZFS filesystem documentation and examples.
ii zfs-initramfs 0.6.5.6-0ubuntu14 all Native OpenZFS root filesystem capabilities for Linux
ii zfs-zed 0.6.5.6-0ubuntu14 amd64 OpenZFS Event Daemon (zed)
ii zfsutils-linux 0.6.5.6-0ubuntu14 amd64 Native OpenZFS management utilities for Linux
# systemctl --failed
0 loaded units listed. [...]