Yvsa hit the problem - both the wood and metal surfaces must match perfectly, and even then the riveting must not put unequal pressure on the grain. Burl is denser, but unpredictable, ie the grain structure is dense beyond most normal grains, but the internal stresses are wildly uneven, and even drilling can cause a piece to split. The handle shape is not a good one for wood. The pommel curves - the wood grain does not. The result is a "tail" of grain which is not supported for any distance, and is more susceptible to normal stress, moisture etc., etc. Fitting and riveting a block to the tang, and then shaping it, relieves the stresses in normally grained wood, and it is prone to cracks. Hand fitting pre-shaped scales is prohibitively expensive, and when they are riveted in place, the handle shape makes them prone to cracking anyway. Uncle's idea of sections is probably the most viable way around this. It would add to the cost, but good, straight-grained Saatisal scales and a horn tail (sorry, it's early) might look neat, and cut way down on problems.