The guys that own their own cars are not Uber employees - they can sign up to be dispatched by traditional radios instead (or as well). So, from my sample in NYC, Uber is way better than existing models for the drivers.
I know I personally wouldn't ever say anything negative about my employer, even though I personally haven't any cause to.
Your driver doesn't know you're deeply interested in tech startups like his employer. Almost all taxi conversations are brief, casual, on the surface. If your next taxi driver took an innocent small-talk question as a chance to air his gripes about his new, better employer, how would you feel at the end of the ride? How are you going to rate that driver? 1 star? Oh shit, now he's canned.
You're getting the only safe answer to your question. For all you know, the drivers are terrified to speak out.