I've used Uber in Hanoi (Vietnam), Al Khobar/Dammam/Dhahran (Saudi Arabia), Bahrain, and Capetown (South Africa).
I had problems with drivers not coming in Bahrain outside of Manama, and I got stranded once. In Manama, and in all the other locations, it worked great. In South Africa, it was safer and more secure than a regular taxi.