Digging can totally be made a fun mechanic. The key is that because digging is so time consuming you'd always be making trade-offs and trying to do it as efficiently as possible, and at any given time there are many different aspects that you need to keep in mind.
Here are some possible features:
* various fundamental different means of actually breaking up the rock, such as hacking away with a pick, explosives, "light a fire by it and then cool it down with water so the heat expansion break it to pieces", "drive a wooden wedge into the rock and put water on it so it swells and breaks a pice lose", etc. Methods along the lines of the last two would dominate. Each method has advantages and drawbacks, and are suited for different purposes.
* you actually have to dispose of all the rock. This would be mostly automated, but you need to always keep in mind the route back to where you can dump things of. It also makes construction of a rail system critical to nay larger operation.
* constant risk of flooding, pump systems, water damage to equipment. Equipment wear and breakdown in general.
* different tunnel sizes, need to constantly add supports, risk of getting stuck in small tunnels, cave-in mechanics.
* Hazzards can be breaches such as pressurised boiling water, acid rivers, suffocating or toxic gas, aquifers, etc. Radioactive minerals can give you radiation poisoning, asbestos can destroy your lungs and so can even normal dust in large enough amounts, cold water and infected wounds can also be lethal if not treated.
* canaries for warning against bad air quality
* oxygen and ventilation. Suffocation in case of caveins. Various means to reduce your oxugen consumption. Being trapped under rubble.
* Light and fire to be able to see or do anything, getting lost and not having an innate sense of direction and exactly how long you've walked, fire consuming oxygen. injuring yourself randomly in por lightling conditions link back to previously mentioned infected wounds.
* Climbing ropes a LOT, and ladders, and elevators, and narrow bridges consisting of just a few wobbly planks laid acros a gap, and more proper bridges.
* Lose rubble, slippery surfaces, rotted ropes and supports.
* Cabin fever and other mental illnesses from being in a lethal, miserable, and lonely environment.