This limits the thermal mass to the slab itself, excluding the ground beneath. However, they are quiet, don't stir up a lot of dust or harbor fungi in the ductwork, and make for a great floor. On page 16 is a heat storage table that lists the BTUs stored per cubic foot of various materials for a ten degree F raise in temperature. Seems to me a good floor would be a concrete slab with a couple of inches of sand to cover your tubing, and tight set bricks on top, with no mortar. Actually it IS baked.
Cement is clay with the water cooked out of it.
Not a bad idea, really except for the pollution. I wouldn't call them rumors exactly, it can happen. But the only time I've heard of it happening is when PB piping was used and run through the concrete where expansion cuts were made. It is then possible for the concrete to settle more on one side than the other and the shear to cut the pipe. The solution is to dip the pipe below the concrete where expansion cuts are made.