As a frequent Uber user in Chicago, I can attest to the fact that Uber often has no available rides for me when I need one. Just this past weekend at a rather busy intersection in Chicago, after requesting both an UberX and regular Uber taxi multiple times, I kept getting texts from Uber saying "we can't find an Uber for you at this time. Sorry for the inconvenience. Please try again soon!"
So the article's suggestion that "customers opening up the app and seeing no rides on the map" would be an existential threat to Uber is currently being proven wrong by Uber's success. That happens all the time and Uber is still doing alright.