Yes, it is. With one that caveat that if it's built 10x cheaper than comparable tunnels, it is. Moving our roads underground is a vastly superior transportation system than we have today. It can be safer, better for the air, better for light and noise pollution, unlock vast amounts of land in our urban areas for parks, etc. I grew up near Boston during the big dig and the city is a drastically different place before…
as i understand it, the high costs of transportation projects are a political problem not an engineering one. well-run subway projects have been completed at $50-100 million per mile, while most such projects in the US are 5 to 10 times that. that's attributable not to sticter safety standards or excessive land costs or public agencies being inefficient, even if those things do contribute at the margins. it's because so many constituencies laden these projects with deadweight costs (lawsuits, foot-dragging, overcharging, kickbacks, etc.).
while his engineering leadership is exceptional and that contributes to lowering costs, elon adeptly sidesteps much of those constituent ladenings, perhaps through careful choice of goals and projects and sheer force of will. for all his faults, he seems to have the right blend of vision, skill, money, charisma, and motivation to bring these projects to completion without a lot of deadweight loss.
so the boring company's technological advances just won't translate to cost reductions generally in underground transportation projects. even if a project uses the boring company for the tunneling, the large infrastructure contractors will find ways to extract their generous slice of pie. we need political/social/economic solutions to reduce these costs, not technological ones.