Most of the closed fist punch to a hard target results in a broken hand some of the time but not all. The statement that a open hand strike cannot injure your open hand striking a hard target is not fully true. Yes if you hit the attacker at the right angle and the right amount of force you will never get hurt by using open palm. But using palm heel at a slightly different angle and targeted at the sternum can break your wrist. The picture bellow is a palm surgery scar after fracturing the scaphoid by using the palm heel strike. Yet the palm strikes are safer but also slower and have less range.
If you do choose to use the open palm to defend yourself be careful and make sure you know what you are doing. Use the part of your palm that is shown down below. You should not use too much force then you can injure yourself. Personally i would use both their are cases that you need to use one and another cases for the other...