Note the same warnings from the UI apply but there is no confirmation on the cli.
Please refer to fwconsole commands (15+)#Backup to know details about Backup module related CLI commands applicable for FreePBX 15+ systems.
To see what is in the backup you can check the manifest. The manifest is returned in json and can be processed through automated methods. Note backups are PHAR files and can be natively accessed through php's PharData class.