The is a very specific risk and it is when a passenger is being picked-up in the street.
In the event that the passenger is attacked (typically young females catching a ride home in the early hours after a night out and a few too many), the victim can usually only provide vague information about the car or driver, which more often than not would remained untraceable.
And I do remember as a young man after leaving a late night event seeing several nondescript cars parked outside the venue with the drivers saying 'Taxi, sir? Taxi, madam?' etc.
It is this very specific risk, and this one only, which is both very real and very serious.
And the message was that a licensed taxi is safer than an unknown car and driver (though sadly there has also been at least one case where a black cab driver raped several women passengers over a number of years).
But with any form of pre-booking - phone, app, etc - the car and the driver can be identified and traced. So this particular risk does not apply to Uber.
Any other risks are far less tangible - you could argue that black cabs are safer, that their drivers are better drivers, etc etc - but ultimately none of this related to the risk of being raped while drunk by a driver the victim can not identify in a car she can not remember.
I don't think anyone can argue that the black cabs are safer, nor that their drivers are better drivers either. The ONLY argument that they have is that they passed the knowledge. Woopiedoo, yes that was great 15 years ago, but not a big deal nowadays.
There is another risk with pre-booking, it is not unheard of at all that licences taxi drivers pickup their car and sign in, yet then give their car to someone unlicensed who does the pickups. At least with Uber you can easily check that the driver is the driver that is registered (identical twins exempted ).