Ever since the first tricycle landing gear fighters came on the scene, research proved it was generally safer to force land with wheels down rather than belly land wheels up. Ditching was different. The RAAF followed this advice with the introduction of the Vampire, Meteor and Sabre jet fighters in the early Fifties. The advantages were energy absorption taken by the landing gear system instead of through the pilots spine as with a wheels up landing. Also with wheels down, brakes are available thereby shortening the landing run. Different with tail wheel aircraft of course where heavy braking on touch down can cause the aircraft to go over on its back.
As with so many flying schools, most instructor school graduates haven't a clue about flying beyond what is taught to them by their own grade three instructors during their early PPL and CPL training. Google might be your friend as they say but very few pilots study flying training history.
In this Duchess accident report, a wonderful opportunity was missed by ATSB to discuss the pros and cons or flight safety advantages of a wheels down forced landings versus wheels up belly landing. A belly landing is fraught with danger on any surface because of lack of energy dissipation and the potential for spinal injury as the shock of impact is directly through the pilots seat to his spine.
Landing gear down on uneven surface may result in one or more landing gears tearing off. But at least in doing so, energy is dissipated that way rather than through the pilots spine at fuselage contact. I believe flying school instructors have a duty of care to inform their students of the facts of the matter.