The Foliage Tool allows specifying a max angle - that prevents trees growing on slopes 
For this, one point on the landscape gets sampled and if the angle isn’t too great, the tree is placed. Works great.
However, for bigger assets this can become a problem. A large rock or a fallen tree need more than a single point sampled. Otherwise, the center of the rock might be placed perfectly fine on the edge of a cliff - but the sides will float in the air. That’s because only the centermost point of the object queries the landscape angle.
An option that samples in a radius around the object would be great. That way, there would be no partially floating rocks or fallen trees.