As my instructor points out (empty hand system) that most people don't "defend" their hands and it is a good distraction technique if you can punch the back of the hand. Anything that can buy you time or get their mind on other things works in your favor. Best case is that you jam up their fingers or break the small bones in their hand. Works especially well when you have someone "pawing" or leaving their hands way out that have to be bridged to get to the body/head. I have used it plenty of times and it has always worked well because unless you have had training where the hands are a target most people don't move them out of the way to a strike directed at them because they don't see it as a threat. But, as others have pointed out if someone is holding them really close to the body then there are other options that may work better. Such as, striking directly to the head/body or striking the arm to temporarily "trap" or immobilize the limb to strike to the head/body.