Question about ebs patching

I am very new to ebs. During the training class, the instructor said that ebs patch cannot be rolled back. So, everytime before patching, make sure to get a good backup of the system. My question is if we are going to restore for what ever reason that patch has massed up, do we need a complete restore of db and apps tier? How is the patch effects? Or, is only partial restore to some file system needed? Please advice. Thank you very much.