True, but there are usually work arounds. I'm wondering if the best way to define a northern slope more accurately would be to animate a light travelling in the arc of the sun from morning to evening. Then render a series of top down views of your landscape, with all other lighting switched off, so you create a black and white mask of where the sun hits the terrain. Then combine say 12 sequential frames of the suns positions, to use as a mask for your snow / planting / weathering areas.