Any updates? Thanks! Looking for some insight!
The technical difference is that an unsafe backup may not contain all data because the Tableau Server is still running and handling requests. For example, if a user is publishing a new workbook and new Extracts to Tableau Server while the unsafe backup is running, there can be no guarantee that this brand new data will be included in the backup because the Tableau Server has not been stopped. Stopping Tableau Server guarantees that there are no changes being made to the data (no publishing occurring, no users being added mid-backup), so the backup file will be a perfect snapshot of all data on the Server at that point in time.
You would use the unsafe option if you are unable to stop Tableau Server to take a "safe" backup. For instance, if you are running a backup script during the middle of the workday and do not wish to create an outage window and interrupt your users. Or, if your backup file is very large, may take many hours to complete, and may possibly run over into normal business hours. If you are OK with having slightly incomplete data (meaning, the data that is being added / modified on the Server at the time of the backup), an unsafe backup should be an acceptable alternative. This becomes less relevant if you run the unsafe backup at a time of day when few users are hitting the system.
Note: You can only use backups made with the tabadmin backup command when restoring Tableau Server data. Database backups made in other ways, and virtual machine snapshots are not valid sources for restoring Tableau Server. For more info: http://onlinehelp.tableau.com/current/server/en-us/help.htm#db_backup.htm