The biggest problem? Bird droppings.
Loose wiring
The natural movement of birds and animals can loosen panel wiring. When connections are compromised, it can have a negative effect on an entire system and efficiency can suffer. Squirrels also like to gnaw on the wires, for some reason known only to themselves.
Birds and other animals sometimes decide to build nests or burrows among solar panels. The new residents then constantly move around the system, increasing the risk of poor connections.