Top heavy is easily countered by putting a sheet of 1 or 2 mm lead sheet under the foam,
If you're using sculpted foam, there shouldn't be an issue with top heavy. I was talking about flat pieces of plywood, those you'd be heaping coal onto to made a good looking load. Those I questioned if they would be top heavy. I don't see sticking 2mm of lead under the foam as a solution in many cases, that would really increase the weight of the cars, perhaps too much.
Of course, there's also the cost and availability of sheets of lead.
There's also the handling precautions of dealing with cutting and handling lead.