Ubers aren't exactly 100% safe and anyone can sign up to be one. One of the main reasons you would order an Uber, would be because you're highly intoxicated and not fit to drive. That's also one of the times when you're at your most vulnerable state.
If you want to look out for your friends, I highly recommend always photographing the Uber driver's license plate (especially if it's a female friend). It never hurts to take that extra step of precaution. Bonus points if the driver sees you doing so.