It doesn't back up everything. Was there a specific thing you were thinking of backing up?
The backup feature is reliant upon the app developer supporting the API to do so. Games should utilize Google Play Games to sync your progress to other or new devices. Other apps should support the API level to allow backup using the Backup menu in the Google Drive app. I seem to recall reading a post from someone that mentioned Samsung may have found a way to disable that capability to force people to use the Samsung Cloud tools.
