I love this style of game, however I have a few issues with this implementation. Though it took me a few moderately enjoyable hours to beat, I think the scaling in the game was atrocious. At the start of the game I had relative ease collecting coal, then iron, and then plateaued for a bit while I grinded to get deeper.
The mission rewards and cash scaling increase too quickly and the cost of the upgrades is therefore too little in the late game but too high in the early game. I had a fully upgraded miner with 3 missions to go, the money from which had come from doing missions/very light mining in between.
All in all a good fun game, but the scaling bothered me. Easy at first, hard in the middle, easier than the beginning at the end.