The real advantage is scaling up the number of machines so you can iterate more cheaply, and add capacity continuously. They can always scale up a design at some point in the future.

As to tunnel exits, the goal is not to have 3 or 4 exits per city but hundreds. Even single lane roads can send vastly more people than any one building needs the problem is the longer the roads the more buildings you need to serve. So, his solution is to have multiple layers of grids under the surface.