And yet, you opted to respond with one.Bah. Anecdotes are not science.
I never use the closed fist anywhere except in the dojang. And the ONLY time I've ever hurt my hand? A messed up block (with a closed fist...) while sparring against 3 opponents. #1 had been knocked out of range. I was focused on delivering a roundhouse to the head of #2, and blocked the kick of #3 incorrectly. The low block was too early, and got in front of the kick, and I caught it right on the 4th knuckle. That was about 15 seconds in, and by the end of the 2 minutes, that hand was a bit sore.
The idea that openhand strikes are less effective than closed is just silly.