From my experience with shimano SPD's and eggbeaters, they will disengage when you fall. But not always before something traumatic happens with you attached to your bike. They are made to disengage when you fall, but that might not always happen that quickly. When I fall, some times the bike detaches quickly and wonderfully from me, other times I'm still half connected to it, lying on the ground. (Not so fun)

Lots of people ride clipless on mountain bikes. I've gone back to flat pedals because I kept hurting myself with clipless in technical sections, your millage will vary.