The current use of tunneling vehicles work, for the most part, fine. I think they still need to tweak vehicles + towed on declines as it still gets squirrelly to control. I don't think you can get a 'perfect' tunnel using polygons and once I make a tunnel with the large drill/paver, it's pretty good for anything else. The overhead can get in the way at times, but it's not a game breaker.
So the real question is, why would i want to use this idea versus what is already in the game?
If you want to sell it, sell the entire idea that you have. Just for the sake of a perfect tunnel isn't enough, in my opinion, to divert dev resources to since there are a boat load of issues that still need to be resolved and should take priority.