The leaked financials[1] seem to indicate it's a losing proposition until self driving cars are relatively ubiquitous.
So whenever it does become financially viable, there's a pretty long payback cycle. And no obvious iron clad moat that keeps competitors at bay...ones that aren't under pressure to pay back investors for years of subsidized prices.
Of course, the leaked data is incomplete, and using unusual accounting, so perhaps I'm wrong.
[1] http://gawker.com/here-are-the-internal-documents-that-prove... and http://www.nakedcapitalism.com/2016/11/can-uber-ever-deliver...