Hearth fires don't go with the other two, and it's pretty disappointing when you've discovered a mine really deep in a cave. I suggest that hearthfires be buildable in either mines or caves, or both. I assume this is intentional, but I can't really understand why. Is it so some mines will go untouched until they're the last ones left?
It makes me sad that I could get metal if I were willing to take a ~30 minute crawl (carrying containers since there can be no smelters in caves) for each trip.