It's really powerful when you're already ahead from having claimed the earlier achievements. It's not feasible to try to rush this if your opponents took 1-4, for example, but if no one claimed 1-4 or if you did, then Coal is a punishingly fast scorer.
In addition, just Coal by itself is 3 factories, and if you have any red 4's tucked that's another icon. That means it is actually surprisingly difficult for your opponent to catch up in factory icons.