I think anyone who’s Ubered much (or driven much, for that matter) can tell you GPS is not nearly as good as knowing the streets. It may be good enough that it’ll take over, but it’s still a lot worse.
GPS is better in that there's accountability. For complex cities, as a driver you can really screw over a passenger by taking a convoluted route. As a passenger, you can watch the driver leave the GPS route (or get a trip report) and know that you've been (or are being) screwed.