I've also read that back blows may further lodge the blockage in the throat, especially if you can't get the person to lean over, and this was a concern for me as well. Whether it's true or not is hard to know, but when your family member is choking and close to losing consciousness, you really don't care about breaking ribs! It's kinda the same with CPR - you don't care about breaking ribs, and it's pretty common for that to happen during compressions.
I'd be interested to know what the potential injuries are from the HM - is the main one breaking ribs?