Someone named WazWaz brought up a fantastic issue on the GitHub, that the Mining Machine is patently terrible. Basically, it is nearly impossible for it to turn a profit.
The main issue is, I believe, not with FT at all, but instead with Vanilla's ore generation method (which I hate, but that's another story). In Factorio, you have huge piles of coal, iron, and copper that you can put Mining Drills on to get a steady supply of ore for a really long time. In Vanilla, ores are evenly dispersed through the world, and physically digging them out isn't worth automating unless the area is massive (such as Quarries and Borers from other mods) or they're cherrypicked with something like a Digital Miner. Even if I divided the part consumption rates for the Miner by 20, it still wouldn't be worth it since you'd have to keep picking up and moving the thing.
One idea that I have to fix this is to make the Miner something more like the Fluid Drill, where you place it down in a chunk and it'll generate ores on its own. It would always consume parts at the same rate; the difference could be that it only succeeds in pulling up a piece of ore x% of the time, giving cobblestone instead otherwise. This success chance, as well as the type of ore, would vary by chunk, giving the player reason to explore and find the best place to set up mines.
To balance it out, what I will probably do is make it so that if too many Miners are operating in the same chunk (alternatively more than one within a certain number of blocks of each other), they will all simply shut down and refuse to run because of interference. Then, the Terraneous Extractor will be more of a direct upgrade, where it can generate any ores in any location, as well as being faster and more efficient (but requiring higher-tier parts).
Thoughts?