I did have one taxi driver (thanks Denver, CO!) alter the terms of the deal on me at payment time. When I questioned why the fare was higher than expected (since it was supposed to be a flat rate from the airport) he asked me how he was supposed to make money if he charged the agreed-upon rate. One of the most idiotic things I've ever heard someone in a business transaction say, and I almost told him to take a hike right there. But, my company was paying, so I figured it wasn't worth the trouble…
I don't think I've really ever had any issues with Uber/Lyft, aside from I wish I could rate drivers honestly without worrying about ending their career. They're honestly not that different from taxis…