The various subtypes of dragonborn in D&D 5e have different elemental resistances (acid, cold, fire, lightning, poison). Tieflings similarly have fire resistance, and I would estimate that a variant tiefling with, say, cold or poison resistance instead would be equally balanced.
However, suppose I want to give a race resistance to one of the physical types: bludgeoning, piercing, or slashing. For example, a dragonborn subtype with slashing as its one resistance, or a tiefling with piercing resistance.
Would this be overpowered?