I can think of no very good reason why robot taxis wouldn't have better coverage than other forms of mass transit. Human-driven taxis are already more versatile in their coverage. Subways and buses have extremely fixed routes. A robot taxi would presumably be able to go anywhere the GPS navigation system is aware of, and existing systems are already vastly more open in where they can go. Check your local subway/bus route maps, then pick any two arbitrary destinations and plug them into google maps.
Google is specifically talking about vehicles
picking you up at your front door. No subway is going to do that, and there's no reason why the taxis couldn't drop you off at the specific front door you wanted to go, as well.
As for costs, there are a lot of reasons why they could be cheaper. Obviously it would depend a lot of implementation, but
subway systems cost billions of dollars and take
years to build. Whereas robot taxis would use already existing roads. So long as they can keep their unit costs down, there'd be a huge cost benefit. Admittedly, google is presently spending
$150,000 per vehicle. In comparison, a New York taxi is a
$20,000 to $35,000 vehicle. It would take an awful lot of savings from not having to pay for the driver to make up that difference. But costs would presumably go down with mass production. It's notable that the current driverless cars are standard drivable cars that are being modified. That doesn't need to be the case. Again, they're specifically talking about the option of making little one and two man vehicles for this rather than the four seat vans being used as cabs. If they decide to make these two-seat podcars, with the usual driving interface equipment like steering wheels and brakes all under the hood, I think it's reasonable to suggest that they could be produced far more cheaply.
Then, once they exist, expansion becomes trivial. A subway, once built, has a very fixed location. Opening up the subway to new locations means spending billions more dollars over years all over again. With robot cars they can easily introduce them in small quantities into a specific city that's suitable for them. 100 to begin with, maybe, in an existing market where taxis are already successful. Simply make fares cheaper, and people will use them. Then add to the fleet incrementally and expand the coverage area. Unlike a taxi service, which needs to train and accommodate drivers and dispatchers and have conveniently centralized parking garages so drivers can return their vehicles then go home, a robot taxi fleet could be far more flexible about its range and location.