For, but only after mining, equipment damage, caveins, military, and seiges (at least) are fixed in a good state.
Mining/equipment damage: A copper pick cannot carve away an entire mountain, even if copper picks can eventually wear away granite. The expense of the tools needed to breach defenses needs to be less than the expected benefits.
Caveins: There is no such thing as a sand ceiling.
Military: A good mining pick is a very poor military weapon. A military pick is a very poor mining tool. Compare a bec-de-corbin with a pick-ax. Then try to do anything wearing armor. (Good substitutes include biker's leather, as it is basically
modernized leather armor, and hard welder's hoods (Without tinted lenses) for a helmet.) Now consider swinging a pick-ax at someone who has a spear pointed at you, in a confined area. Tunnelers are noncombantants.
Seiges: Tunneling should be pretty far down the list, after other unimplemented seige styles: Attacker constructions and deconstructions, sabotage of food/water supply, intentional flooding, and taking one look at the fort and leaving should all be implemented sanely before tunneling.
While there will be implementation problems if it is ever implemented, including framerate and calculation issues, those need to be addressed seperately. I don't want to see cats changed because of framerate issues, even though I would like to be able to
alter them.