The changes of temperature are seldom a problem in a car, but the bikers generally have a much narrower comfort zone. Too cold and not only does the riding fun disappear, but under the freezing point ice and snow render the road too dangerous to ride on. Too hot and the biker needs to transport far too much water than he or she can carry, in particular when riding in the desert to be able to cope with a possible break down. Even a car needs to be properly prepared to be able to cope with temperatures below -30 celsius, which are routine in North Siberia.
Rain is a much more serious problem, and not because the bikers generally aren't too fond of getting wet. The problem arises in developing countries which are affected by extreme rains, such as during the monsoon or the rainy season of Equatorial Africa, when the roads can become totally unpassable. If there's a deviation on a tar road, then it is usually possible to continue the trip, one loses only the pleasure of riding or driving on a piste. But when there is no such alternative, such as on the famous Mamfé-Ekok stretch between Cameroon and Nigeria, the overlander can become stranded for days or weeks.