The basic rules for vertex manipulation are simple, though sometimes a little hard to keep.
First, a brush must always be convex in order to be processed correctly by the compile tools.
Second, every face must be completely flat. The opposite of this would be a bended face, one with two normals (two directions, so to say). Hence the co-planar face error.
Third, there may not be two faces who have the same normal, that is, the same direction. These should be made one face, or you should use multiple brushes in such occasions.
For complex or odd faces, using triangular faces is a safe way to avoid most problems.