Drying trays, netting to protect against bugs, and the fruit are the only investment. Another possible advantage is the sun's sterilizing effect caused by ultraviolet rays that may slow the growth of some organisms. Sun drying is dependent on the weather. If it is sunny one day and not the next, you have to finish drying your fruit by one of the other methods before it spoils. Also, when it cools at night you have to bring the food inside.
Spoilage can occur while the fruit still has enough moisture for microbial growth.
Another disadvantage is time. What would take 6 to 8 hours to dry using another method, may take 2 to 4 days in the sun. Solar drying is like sun drying, only better. With a shorter drying time, microorganisms have less chance to cause spoilage. If you don't want to buy or build a solar box, the back window ledge of an automobile where the sun shines through can be used as a solar dryer.