I think the biggest threat to Uber is unionization (or other labor organizing).
Re:In Search of Uber’s Unicorn
by 7Figures2Commas on Tuesday October 28, 2014 @07:30PM
(#8524412)
While you can join a union as an independent contractor, independent contractors who are members of a union do not have the same bargaining rights and protections as employees who are members of a union. So here too, the classification of Uber's drivers makes a critical difference.
Most threatening to Uber's long-term viability are putative class action lawsuits, such as one alleging that drivers were misclassified as independent contractors[1]. If Uber is forced to reclassify its drivers, it could literally upend the economics of Uber's business overnight.
Because of the legal and regulatory challenges it faces, I think Uber is actually in a far riskier position than Groupon ever was and Uber's valuation seems to indicate that investors have too significantly discounted these risks. At this point, I actually think Uber might for some investors prove to be a binary investment. That wasn't even the case with Groupon, which still has a $4 billion valuation despite its decline.
[1] http://uberlawsuit.com/