I only tried two routes so far (Northern Virginia) and it did OK. But you can tweak these routes the same way you can tweak riding routes by just grabbing and dragging the route line. It took me five seconds to adjust a route from a road that I know I don't like riding to some neighborhood roads including a small pedestrian-only causeway.

Washington Post article about this said that it gave routes directly through Arlington Cemetary where bikes are not allowed (oops). Makes me think that some of the Bike routes they give are likely based on the Walking routes. Hopefully with feedback from users this feature will get better over time.