Kenneth chung, frowns on anything other than sand and he has been practicing for 40 years, without becoming a cripple =)
Opinions vary between Sifus, I guess, as does the tolerance of the bones of the hand for impact. My advice came from a senior Australian MA who has also been training for over four decades. Not with the same profile, perhaps, but I'd imagine equivalent skill and knowledge levels.

Does KC hit the wall bag full power all the time? Did he do that when he started out or did he work up to it? What do you do?