It can be very lucrative money, but be very careful if you are considering driving for Uber. Aside from the safety issues people have already mentioned, there is another MAJOR concern that many do not know about. This happened to a close friend of mine.
I do not know the finer point details, but driving for Uber could void or severely impact your vehicle warranty because of exclusions that may be written in against using your personal vehicle for commercial purposes such as a taxi-type service. My friend was driving for Uber to supplement their family income (he was out of work at the time and was using Uber to make ends meet in their single income family). It was a relatively new vehicle - a 2015 model Mazda (I may be off by a year or two) with no mechanical problems he knew of.
It broke down during a family trip out of town and had to be towed to a repair shop. It was some sort of engine issue which would have otherwise been covered under warranty but because the car had been used for Uber it was not covered and they could not afford the repairs (several thousand dollars I am told). End result = no more Uber income, no more family vehicle (it was eventually repoed from the repair shop), an absolute nightmare all just from trying to earn a few extra bucks with what was otherwise a reliable car.
So if you are considering Uber, READ ALL the fine print in both your car insurance and you vehicle warranty before making the plunge. My friend was not a heavy Uber driver - just a little bit here or there, but even that small bit was enough to void the warranty.