How even the temperature is at the floor and how readily the heat transfers from the tubing depends on the spacing of the tubes and the conductivity of the floor material. Pea gravel and sand are not great conductors because they contain so many air pockets. Adobe, mud, terratile are good conductors, though not as good as concrete. The noble reason not to use concrete is that there is a high embodied energy in the cement resulting from it having been baked in a kiln. Finally, modern tubing used for hydronic flooring rarely leaks.
Tubing is flexible, and is laid without joints.
The only leaks I've ever heard of came from freezing. Our solution has been relatively shallow slabs or gypcrete, concrete, adobe, etc. over insulation. 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.