Had one of the internal 3TB drives fail on the new server which was part of a ZFS raidz. At least ZFS is working fine, the drive was marked as faulted but the system was still running in degraded mode.
Took out the drive and replaced it with a 6TB and after 4 hours of resilvering, the system is back online.
So yes the raid thing does work quite well. And I will probably start replacing the 3TB drives with 6TB drives as they fail till they are all replaced and then a resize should give me double the capacity.