I don't know, Batik. I would imagine the rice would be less flammable than the wheat ones but I really don't know for sure.
I used a wheat one for my neck for a very long time. I also had one that had some type of beans in it. Not sure what they were. Both were bought at market stalls if I remember correctly. They were really useful at the time. I wasn't fond of the smell of the wheat one really. Also worried about bacteria, but I tend to worry about those sorts of things more than most perhaps.
The problem with the one involved in the fatal fire was that she was using it as a bed warmer
and it was obviously too hot and once it was under all the bed covers it ignited.
Some of the heating bags come with herbs and oils and all manner of things in them. That would add to the fire danger I'm sure. Not all microwaves are the same either and they tend to create hot spots. People sell them in markets here everywhere. The material used for the cover is something else that could add to the fire danger. Heating them in the microwave with a little water in there might help too. Shaking them around after heating might help as well.
This inquest here will hopefully make it mandatory to add a warning label so people are reminded of potential dangers. There are lots of things to consider which I actually had never ever considered when using them myself.