I like breaking boards. I think it is useful. The rebreakable boards just won't break if your aim isn't spot on. I have seen people pound away at these things unable to break them not becasue of power but becuase of technique. You do get a sense of proper technique when you have to hit a board. I don't know about 8 or so boards but 2 to 4 seems reasonable once or twice a year with a good rest in between.
I agree that attempts at conditioning could lead to the issues discussed but that is a personal choice and it doesn't seem to me that there is enough information on conditioning. If your idea of conditioning is simply pounding a piece of concrete well what do you expect.
I have seen labourers, mechanics, etc with hand issues when older. I have seen many people who never touched a board and have very bad hands in thier sixties.
Now children should not break anything period. Children have undeveloped growth plates between bones and any damage to these can case the bones to grow to fast or too slow.