I think this is the key:

8. Traditional business analysis raises serious doubts about Uber valuation and growth potential. If one applies typical business school analytical criteria (focused on issues such as technology and process innovation, competitive advantages from sustainable cost efficiencies or superior customer service, the coherence and scalability of business models and the potential for future market growth, and expansion into other complementary markets) it is difficult to understand why anyone would see Uber as a likely candidate to achieve the stratospheric long-term growth that would be needed to justify the huge valuations that the company and its supporters seem to expect. Uber clearly appears to have won a degree of acceptance among customers with high disposable incomes in large, wealthy cities, but that sheds no light on the plausibility of Uber’s ambition to radically disrupt and transform the car service industry and to become that industry’s dominant worldwide player. The Uber business model appears to involve higher vehicle capital costs, higher maintenance costs, poorer capacity utilization, and less knowledge of local market conditions that existing business models, and would likely also face higher labor costs throughout the years until its hoped for industry shakeout had been achieved. The potential advantages cities by Uber’s publicity appear to be ones that competitors could readily match (smartphone apps) or that depend on labor exploitation (drivers willing to absorb vehicle capital costs and risks without full compensation) and/or regulatory arbitrage (failure to pay necessary insurance and licensing costs that its competitors must pay). There is no clear link between the tens of billions in financial value and any tangible new economic (efficiency/service) value that Uber is creating, and there is reason to be concerned that some of that value depends on the destruction of existing economic value (safety and consumer protection) and wealth transfers from labor. The value of today’s (highly fragmented) industry primarily resides with the people that provide capital (vehicle) financing and manage how those assets are utilized in the marketplace. Uber’s only contribution to the industry value chain is booking/scheduling software, but it is attempting to capture all of the value contributed by those financing and operating people. A major portion of Uber’s prospective valuation assumes it can wipe out existing suppliers and achieve a sustainable market dominance. This would not only secure its ability to capture value currently held by labor and other parts of the business/supply chain, but would facilitate significant wealth transfers from consumers through pricing power and the ability to further weaken safety and consumer protection rules. But since there does not appear to be any basis for expecting that Uber could achieve enormous sustainable competitive advantage over existing operators, there is no apparent reason to expect any of the major market disruption/dominance aspects of its valuation to be achieved.

The bull case as articulated by Business Insider is the $10 billion run rate:

http://pandawhale.com/post/53520/uber-revenue-10-billion-run-rate-in-2015-according-to-henry-blodget

Also articulated by Vox:

http://pandawhale.com/post/45191/why-uber-just-might-be-worth-it-at-18-billion-vox

The bear case as articulated by 538:

http://pandawhale.com/post/46217/uber-isnt-worth-17-billion-by-aswatch-damodaran-at-fivethirtyeight