Iron is definitely the toughest ore to get, AND the one you're liable to need the most of. Many swamps have no crypts, and using the wishbone in the swamp is hardly worth the effort. It's a real slog finding the scrap piles in a swamp, and half the time they're under the water line. It's a real PITA to mine the whole scrap pile without creating a Pond of Death- you start swimming, lose your tool/weapon, can't get out, and get murdered by the mobs that inevitably jump you while you're mining. All for a few piddly scrap irons.