Eventually they will add hidden and recessed doorways (probably). I am betting you can make a door out of rock that looks like a wall with a hidden button or lever somewhere at some point in the game (maybe not at launch). I am thinking that if I can make a sweet tunneled mine, that I can trap it up and have a hidden entrance or secret invisible rooms within the mine for personal use in case people wander in. We must defend the mine!

Once I finish draining the resources I can find or reach the edge of my land anyways.

From my understanding, Tunneling, like Building will take some thought, planning and effort. I believe you can make the tunnel as large as you want or can in certain circumstances, or as small as you can. I don't see a underground tunnel for the Kypic being the same size as the tunnels used by the Hrothi.

First off, from your Hrothi tribe description:

Unfriendly

Kypiq - The nature of the Hrothi's underground civilization makes the short tunnels inhospitable to the taller Tribes of Mann. The need to duck or crawl dramatically slows their pace, making them easy targets for the Hrothi to deal with. This provides their civilization a natural defense against most of the other tribes, that is, all but the Kypiq. The Kypiq's ability to move around the Hrothi's tunnels unhindered makes the Hrothi nervous, and the natural curiosity and propensity toward shiny things makes them untrustworthy.

That says to me that tunnels are size dependent, and that the Hrothi tunnels are naturally too short for all tribes but Kypiq.

As far as tunneling to exploit minerals, I would hope that mineral veins are a thing, and it would be really cool if the size and shape of the vein had a sensible geological origin. For example, veins that are buried alluvial (placer) deposits with noble native metals like gold or copper, or hydrothermal minerals like sulfides (cinnabar, for example) trapped between layers of impervious silicate rock.

The Hrothi would obviously be favorites to exploit any buried veins, but of course we also know that other sources exist. The bog iron of the Dras is one example, and I give a few other possibilities in the Minerals link in my signature. Most of all, I hope to see abundant variety of resources of varying quality that encourage many different approaches and playstyles.

I plan to go Waerd and expand the territory underground (because we live in an inhospitable environment), but I am prob going to table such things until I have a solid grasp on what Waerd do (akin to objectives and what their daily life is) and research on tunneling is available for the Waerd.

The Waerd also live underground and there much taller then the Hrothi, so height of tunnels will probably be adjusted for different tribes.

I've already started planning the tomb of my first character, but aside from that i'm not going to say anything on the topic. :p. I'm sure that height can be adjusted, but in general tunnels will be hrothi sized.