Driving on the salt flats is permitted during the dry season but not during the wet season when mud and water pools exist. Remember that whatever time of year you visit Uyuni, driving on the salt flats is most suited to experienced drivers. There are many cons to driving on the salt flats, such as the remote nature of Salar de Uyuni, lack of cell service in case of emergency, and the damage driving on the Bolivia salt flats can have on your vehicle. Moreover, you'll have difficulty finding a local rental company at Uyuni. So, unless you're already on a larger road trip, you'll need to factor in the cost of renting outside Uyuni to then drive in. With all factors considered, renting a car can sometimes be quite costly, and, depending on your travel plans, joining a tour might be the more cost-effective option.