I suppose you won't hear much from those where it had worked (smiley).
As far as I understand the technic of laying wood onto a screed that acts as an UFH the problem lies not in the cracking of the timber itself, called splitting, but in losing the touch between the timber and the screed. Timber "lives" i.e. it moves. The movement depends on moisture and temperature. And since the screed has to be hot, not just warm, to get some heat through the timber, the difference between the temperatures at the bottom and the top of the timber can be too high to protect against warping. So an artificial floor on top is the only chance to get out of that problem. No timber. The next point , after solving the technical problems, is the energy efficiency of such a construction - but you know my opinion about this already.