The ZFS file system provides data integrity features for storage drives using its Copy On Write (CoW) technology and improved RAID, but these features have been limited to storage drives previously.
If you have a drive failure, utilizing RAID or mirroring will protect your volumes, but what happens if your boot drive fails? In the past, if you used FreeNAS, you had no option other than having your storage go offline and remain unusable until it was repaired, and the ability to mirror was only available in TrueNAS, which utilized the underlying FreeBSD code.
In older versions, the FreeNAS and TrueNAS boot drives used the UFS (Unix File System), an older file system that does not include the advanced data integrity features found in ZFS. This has recently changed on current versions of TrueNAS and FreeNAS, and now ZFS can be installed on boot drives using the menu-driven installer via a simple interface.