« Is there enough boreable space under our cities that this is doable? I know Musk says he wants to go deep, but even at very deep depths you run into problems of sand, boulders, geography, etc, etc. »

Certainly out here in a much more karst [1] part of the country (I'm in Kentucky specifically) there's just about no way, from what I read, to bore under our cities without threatening the geologic stability of just about everything above it, because our cities are already essentially built "on top" of caves, caverns, and limestone hollows. Sinkholes are a real problem with sometimes deadly effects.

Tunnels in general here can be very expensive to cover survey costs and insurance. I am not a civil engineer, but I would assume there are probably maintenance tasks and costs that differ for this sort of geology.

[1] https://en.wikipedia.org/wiki/Karst