Also, regarding barriers to entry, Groupon and its competitors felt the need to hire all those sales reps and writers to put together deals. They may have been able to scale more efficiently, but all those hires were certainly a barrier to entry, to say nothing of their exorbitant land grab marketing budgets.
Also, I think the Ponzi scheme rhetoric is way overblown. You could argue that its investors were sold an over-hyped bill of goods, but Groupon's core product was/is sound. If they can scale down their head count a bit, maintain market share with only a maintenance marketing budget, and hopefully expand into local ecommerce, they can still do well. It just might not be a 30 billion dollar business.
Finally, Uber has gone from a near standstill to disrupting a major industry in record time, with almost no experience in the industry. As other commenters have pointed out, one of Uber's chief competitors is now Hailo, a rapidly growing service built by people with tons of industry experience and connections. I don't really see insurmountable barriers to entry here for their competitors, especially given that Uber's detractors would argue that its main competitive advantage has been to simply thumb its nose at what should have been its own barrier to entry: regulation.