Biggest risk: Intermitent and unpredictable energy output. Weather can block the sun rays for weeks or months (as in a monsoon season)
Second biggest risk: Competition from other energy sources when grid-connected. If the price of energy gets lower (due to the suddenly lower cost of an alternative energy source, such as coal, oil, gas , hydro, wind etc.) before the end of the payback period for a Solar PV
installation (which might last decades), that solar PV installation might lose money for the investors who cares only about the bottom line. A corrolary of this risk is the possible end or reduction of subsidies.
Third biggest risk: Rapid advances in the PV Energy technology itself which might render an installation a dinosaur before the end of its payback period.