My Stealth Compost Piles are about 10" into rocky soil under wood decks in this semi-arid environment. The compost is watered every time the lawn is watered.
Since they are under decks, one could say that they are both in the shade but that shade is compounded by location. The chicken house deck is on the north side of the building. The greenhouse deck is on the west side.
Really, the chicken house deck receives only morning sun during the summer. The greenhouse deck has only a little more sunlight because the carport roof is nearby. However, there is a noticeable difference in how fast material decomposes with things going more slowly in the northern exposure.
Still, decomposition happens. It is very likely that the soil insulates the material in the winter. Everything freezes on top (the pile is above ground level by the end of the growing season.)
I'm certain that decomposition would not work out the same in different environments/soil types. However, just considering shade or sun, it probably makes only a little difference. Moisture, air and ambient warmth are more important.
One thing -- compost has real value. Despite still harboring seeds, my compost is better quality than the "compost" that can be purchased and is available here. It doesn't have to be that way but commercial standards sure seem to be low.
Steve