When referring to storage being used for backup storage by your UEB:
Thick lazy or eager zero is preferred. Thin provisioning can cause IOWAITs while the disk expands, and could allow for accidental over-provisioning of a storage device. This would lead to an out of space condition.
When backing up other virtual machines with your UEB:
Backups of thick provisioned disks will take up slightly more space and time compared to thin provisioned disks. This is due to the need to process open space on the thick provisioned disk, however due to the compression and deduplication functionality within the Unitrends product, most of this space will take little to no space on the Unitrends device as there is usually no data in it.