John K Jordan
What I don't like is when fill won't doesn't expand and contract with the wood movement. I've seen so many things filled with metal powder, resin, turquois. Many times with aging the hard fill will be enough proud of the wood to feel the sharp edge with a touch.
This, of course, depends on the fill material, binder/type of glue, wood type, wall thickness, initial MC and final environment, grain orientation in the area, the size of and perhaps the depth of the void, and maybe some other things. Maybe that's why I've had better results filling voids with wood and bark. Besides, I like the look!