Maybe some people with more legal knowledge than me can chime in, but there are limits to Hold Harmless agreements I believe.
I seem to remember my wife talking about this before, but for instance, say I run a flight school and have you, a CFI, sign such an agreement. So you fly my planes and notice several safety issues, which you report back to me. I tell you "I'll take care of them", but I really don't. A few months later, a prop goes flying off and severs your arm (I'm using my imagination here). I think I'm safe because of the agreement. Court doesn't see it that way, as I cannot sign away responsibility for gross negligence.
I think they're more applicable when, say you're playing paintball and you take off your mask to wipe your forehead, and get shot in the eye. If it's within the assumption of risk, I think that's when it's binding.