another remark: due to the behavior of "keep going till you have fuel", the player could find themselves in the unhappy situation of having the inventory full (and doesn't matter if you walk away, you'll keep getting wifi stone in your bag) and you can't pick them up to stop them from digging cause, well, your inventory is full! :)
right now it's quite OP - they default to digging north, so I make a 1-borer tunnel going east or west, then place a bunch without having to set them up, and fill them all - 4 chunks on 10 coal approx, it's 20 chunks on a full coal load - user discretion advised
also: they can "walk" through other objects (I tried putting power poles in front of them, they kept going), they will also temporarily block the player if you stand on their path, till you're out of the tiles they occupy - they can walk on water, there's really nothing stopping them other than running out of fuel, and that's just coal - what with rocket fuel? ehehehe