Good thought. I've no experience with them over an open fire without a pot or other container. Boil bags are made to be boiled in a pot, about 212 F, so a flame would probably melt them. The oven bags are made to resist oven temperatures, but not to come into contact with flame or heat elements, so it'd be interesting to find out how they would deal with a flame.
Obviously, they work as intended when used as intended. We've used them to boil foods in a pot, and used the oven bags in a dutch oven with good success, saves on clean up time.
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"Most men take the straight and narrow. A few take the road less traveled. I chose to cut through the woods." ~Unknown~