I think you are jumping to an extreme puffadder. For most breaks, very little conditioning is necessary until you star breaking above 2 inches of wood with the hand and maybe twice that for the feet. Most people breaking one-two inches of wood with a punch or a kick will not need any extra conditioning as long as their technique and commitment are correct. It is the job of the instructor to supervise breaking properly and make sure that injuries do not occur.

In my experience, I have seen the odd injury due to breaking, which puts me off doing it somewhat, but I still think it is a relatively safe practice. I just find other types of training to be more effective at developing the same qualities.