There are two main problems:

- Big upfront costs (solar lights, particularly high performance ones, can be quite expensive though cost is coming down)  

- They obviously depend on sunlight so their performance goes down in long period where there is less sun

If you are in a region which has lots of snow during the winter then the solar lights will stop working if the panel is constantly covered.

Obviously solar light will not work where there is a lot of shade eg under trees etc. I am always amazed how many people put solar lights where 70% of the day is shaded, then complain bitterly.

Low temperatures and gloomy weather can affect the battery lifespan of the solar lights. When the day is short in winter and autumn, the solar light is less capable of charging fully.