Though I will come out and say that stacking resistance probably isn't broken- with a caveat. There's enough different damage types a player can be subject to that even if you had triple resistance to say, fire, it shouldn't become an issue all that often unless the game is somehow fire themed (trips to the Elemental Plane perhaps?).
After all, monsters can be flat out immune to damage types, but it's not as if players can be (as far as I know, outside of maybe things like shapechange or true polymorph).
The caveat, however, is bludgeoning, slashing, and piercing. These types are fairly common, and any Barbarian can access resistance to these via rage. The designers were very careful to not allow a Barbarian to stack rage with even heavy armor mastery.
It hasn't come up yet, but if someone decided to play a Tiefing Barbarian in a game (Karlach fans unite!), letting their rage and innate resistance stack doesn't sound like the end of the world, and I'd likely consider it.