I prefer wheat packs as I mostly want them on my back and lying on top of a hot water bottle is a recipe for a wet bed!
It's easy to prevent wheat pack fires - just pop a cup of water in the microwave when you are heating the wheat pack and it will be safe. Or you can use cherry stones instead of wheat, which are less flammable, though also less comfortable!
The wheat packs can also be put in the freezer for a cold compress for acute injuries as well.
I do still use a hot water bottle from time to time as I find them more comforting to cuddle if I have a stomach bug. I also have a mud pack that is very good to use for back pain.
My understanding was that wheat packs can catch fire while you're heating them in the microwave, but are otherwise safe. At least if a fire starts in there it should be easy to put out (just leave the door shut and it will run out of oxygen quickly). I have a couple of wheat packs. I found them really good when I was in labour and when I had gall bladder pain.
The ONLY reason that incident of fire occurred was because the senile old woman heated wheat pack for far too long at a high Temp. in the M/W oven!
Just FOLLOW the directions that came with it, & you'll not have a problem. I heat mine about 10-20 seconds LESS time than recommended. Putting a glass of water in with it is 'false' confidence. Bag could still get way too hot.
Used to use HWB on stomach with period cramps. Was THE only thing that gave any relief! Tried to avoid medication when possible, but at times SO bad, used to have to resort to pain killers!
I've only ever use a HWB I'd heard of women getting badly burnt with the heat packs when they've taken them out of the microwave, & a couple have exploded, so I've been to sceared to buy 1. I always wake in the middle of the 'night' to use the loo, so I then refill my HWB! Of cause my HWB always has a plain pink or a patterned pink cover on it. Just squeeze them a little to let the air out, before putting in the stopper, & don't overfill them! I fill mine with boiling water to the 3rd line from the bottom, on the jug.