I noticed that Ploi doesn't really report if something goes wrong when creating backups. In my specific case the backup destination path didn't exist which resulted in the following error message
Uploading ZIP to SFTP server..
cd: Access failed: No such file (/sites/example.com)
Done uploading..
It would be nice if Ploi could notify if something like this happens.
PS: Can Ploi automatically create the destination path if it doesn't exist?