If the engine does fail in a single engine retractable you are almost always going to be better off with a wheels up landing.
Another myth; this time disproved my medical evidence. Forced landing wheels down in a tricycle landing gear type, allows the landing gear assembly to absorb the energy of impact thus minimising spinal injuries especially on rough terrain including rocks. Moreover depending on terrain wheels down means braking action is usually available thus reducing the distance travelled.
All this came about when in the late 1940's,tail wheel types such as the Mustang, Typhoon, Spitfire and even the Lancaster type bombers were around. Experience showed wheels down forced landing generally resulted in the aircraft tipping on to its nose and worse still going inverted.
The introduction of nose wheel types such as the Sabre, and similar jet fighters found the relatively fast touch down speeds in a belly landing in those types resulted in severe spinal injuries among pilots as impact energy was transmitted directly from the fuselage to the pilot's spine. Trials proved that having the undercarriage lowered meant the energy of impact diverted instead through the wheels and undercarriage structure, leaving the pilot less susceptible to spinal injuries.
Wheels down also meant brakes may be available to shorten the ground roll in some circumstances