This is a balance concern regarding the different allegiances (kingdom vs empire). Metal is extremely important mid-game, but this specific concern only applies to late-game.
Empire has:
Smelters - increases metal output in the city by 25% (one per city)
Kingdom has:
Smelters - increases metal output in the city by 25% (one per city)
Blacksmiths - increases metal output in the city by 50% (one per city)
Mining Guild - increases metal output in the city by 100% (unique building)
Unless I'm missing something, kingdoms will have access to much more metal late in the game, when the player has had time to research and build these buildings.
