Depends what portion you are talking about. It's not one tunnel, is't several tunnels constructed using nearly every soft-ground tunneling technique available.
When the NYC tunnels were created, they were in virgin ground. Much of the boston tunnels are under existing highway,partially in fills, partially in clays that woudl not support you if you were to stand on them, and constructed all while maintaining surface traffic.
Yeah, there are some serious screwups, and political crap, but the fact it could even be completed is amazing if you took time to study the entire project.
If the final price were known at the start of the project, would the project have gone forward? What offends most people about this project is the insatiable appetite for tax dollars.
And don't go saying nobody could have known that the costs would escalate so much at the beginning of this thing. The overruns were built in from the git-go.
The failures here are political failures.