Mmmm.......FreeNAS IS my back up, first line of data is at a Windows machines sync to a Synology and then rsync to a FreeNAS. Couple of months once i plug in 1-2 external drives to the Windows machines for File History. Data is about 1.5TB.
Others stored at FreeNAS i actually don't back it up ie movies, rip media (which i copy to an archive) and data stores(working on a back up) which is about 20TB