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 player character 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?
Best Answer
Depends on the race's other perks, but probably yes.
Bludgeoning, piercing, or slashing damage are the most common types of damage in the game. Nearly all monsters have at least one attack with one of those damage types. Barbarian's are so resilient because their rage makes them resist this.
If you just replace a race's "Resistance to Acid" by "Resistance to Bludgeoning", it's a decent power-up. It might not be OP, that would require a race-by-race analysis, but from my perspective, if I had to choose between resistance to one of the elemental types and one of the physical, I would ALWAYS choose one of the physical to resist to (unless I was a Barbarian). And having a choice that seems to always best another isn't good design.
If you lessened one of the race's other benefits, it would probably be balanced, but again, this requires a case-by-case analysis. If you intend to explore this further, I'd pose a homebrew analysis question on its own.