Mating metals must never be of a like hardness as this will result in fast heat up and rapid cold welding of mating parts. Norman cycling may cause excessive friction. Components rubbing together may rapidly cold weld. Therefore you must select the best and least complex surface treatment that can make a low cost metal perform its intended function longer and better. You also must consider load bearing strength for the components collectively.
Weight is another major consideration.
Some metals or alloys may have greater strength, last longer or perform better. However, the weight factor can be a major problem. Machining or fabrication costs are the major consideration for product designers. Heat treatment or electroplating can cause failures from hydrogen embrittlement or stress fracturing. Machining can induce case hardening or surface stresses or ruptures in the surface.