The clouds all have normal maps (materials), which means they will appear to be 3D when you have Advanced Lighting switched on in the Graphics settings of your viewer.
Their land impact is only 4. They move into place by themselves (you can lift or lower if you're not pleased with their position), and you can use them both at ground level and around sky builds. They combine very well with large version 6 sky landscapes and with version 6 sim surrounds. However, they cannot used in mirror boxes, since the clouds will be outside them.
Their textures use blended transparency. This means they will interfere with textures in your build that use the same kind of transparency, like some trees and windows (alpha sorting problem). Landscapes Unlimited version 6 off-sim landscapes, however, use masked transparency for their trees. There is no problem with those.