[larsga]

It's not a choice of TBM vs cut-and-cover. You also have drill-and-blast. Which method is better depends on a number of factors, including depth and the state of the soil/rock you're going through.

A problem with TBMs is it can be difficult/costly to secure the tunnel sufficiently against water ingress. If you mess up that one the cost of your tunnel can easily double or more.

Here in Norway we recently built a railway tunnel with TBMs (Blixtunnelen) and it's having problems with water ingress. A fairly mild problem relative to how bad these can get, but it's enough that the tunnel constantly has to be closed for repairs to the railway infrastructure due to water drips.

[bluGill]

There are many options with different compromises. Cut and Cover comes in many different versions (some of them you build the cover first and then dig under that). However I'm going to double down and suggest that despite the disadvantages everyone else has raised the overall much lower cost of cut and cover, combined with the advantages of a shallower tunnel, makes it the right answer for a lot more situations. Most people claiming to need something else for some local feature are wrong - they can work around that other objection and be better off. Note that I'm not saying the objection is invalid, just that it should be worked around.

[Symbiote]

Are you a tunneling engineer?

That over 90% of projects are using TBMs strongly suggests they're usually the better option.

[bluGill]

They are the political option. They disrupt the surface the least and so there are less political objections even though they cost more around the world.

Tunneling engineers do what the customer says. They will tell you what the options are and what they cost then let the customer choose the evpensive option if they want. Their job isn't to choose between options it is to eliminate the impossible (unsafe) ones and let the customer choose the pros andecons of the rest.