It is perfectly safe to back up QuickBooks with Jungle Disk's Backup Vault feature (available in all versions of Jungle Disk).
However, we do not recommend saving your QuickBooks files to the Jungle Disk Network Drive, Sync Folder, or Legacy Backup features due to the risk you'd be placing this QuickBooks data in. These Jungle Disk features and the QuickBooks data files are not designed to work safely together, so storing that data in the Network Drive can result in loss of progress and perhaps even file corruption.
So if you are looking for a way to share your QuickBooks data across multiple computers, consider signing up for QuickBooks Online, offered by Intuit (the creator of QuickBooks). Or contact Intuit to find out if there are other solutions available that work better for your needs.