simsalabim,
Mass balancing gives equal mass about the hinge line. Flight control surfaces MUST be "mass balanced" to avoid in flight flutter and, simply, destruction of the airframe.
Obviously, the smallest physical mass of "stuff" which will balance is best, hence dense metals. As it happens, depleted uranium is more dense than lead and can be protected, or more to the point kept out of contact with people like us, by proper surface treatment. It is not a good idea to work the depleted uranium using any sort of tools unless you know what you are up to and are properly protected.