The Uber system works on a rating system where every driver starts with 5 stars and is rated by the customer on their performance. In some cases the driver may lose their rating very and if it drops below 4.5 they will be de-activated. Uber will give you the chance to be re-trained however some drivers never make the grade.
This is why we recommend that you rent for at least the first few months. Remember you can not return the car you buy to the dealer, but you can return the rental car if this happens to you.