I am testing the mining units on Iacobus.
I have three tiles, 1 with 34 ilmenite the 2nd with 30 ilmenite and the 3rd with 16 ilmenite.
My thought was a total of 80 liters of ilmenite per hour.
But no, it's 34 liters and the 20% adiacency and max skills make it only 46.92 liters per hour.
My point is that you cannot put a mining unit on all three tiles and remove the ore from each tile.
I think that the adiacency bonus is a punishment in some cases because you do have to pay the 1 mil taxes, but you get much less value in return.