It's complicated and I'm not an expert, but I understand the main risk with TBMs is running into a type of rock it can't handle and suddenly having a multi-mill

It's complicated and I'm not an expert, but I understand the main risk with TBMs is running into a type of rock it can't handle and suddenly having a multi-million dollar paperweight wedged in a very awkward location. They're also not well suited to really hard rock, which requires frequent cutter replacements. In this case, the tunnel is very long and deep, magnifying the unknown territory risk. Also, the bedrock in most of Scandinavia (and, I presume, Stockholm) is both very shallow and mostly granite, which is extremely hard and thus not a good fit for a TBM. Source: My childhood in Helsinki was punctuated by the periodic sound of blasting rock as they cut out what was supposed to be the new central station. Overall, Helsinki is basically granite Swiss cheese due to parking garages, pedestrian tunnels, basements, bomb shelters, military tunnels, the metro etc and AFAIK all of it has been drilled and blasted, not TBM'd.