I'd hazzard a quess that there's a slight difference between the reheater packs you're talking about and the reusable heat pads sold in outdoor store onlyu becasue I noticed from your link they sell a face mask version. I know from experience that the reusable heat pack I have would be intolerable on the face as it gets to hot. I put mine in a sock if I need it becasue it's too hot for the hands.

I carry a couple to augment the chem heaters. Like others have mentioned, they heat hot and fast, and cool down fast. So they are real good at getting a jump start for warmth, perhaps even in an emergency tucked in the armpit, but not for providing long term warmth. I've used mine a couple of times to provide quick warmth to a scout who's sleeping bag isn't keeping him warm enough. I also provide an extra blanket or summer bag (we keep a few cheap ones in the troop trailer) for a "permanent" solution. But the quick heat thwarts hyperthermia when combined with a warmer sleep system.

Since they cool off after 15 minutes or so, they aren't much use for all day or night heat. Oh, they are also VERY cool to watch as they crystalize and make a good intro to the chemistry topics of endothermic and exothermic reactions.