> could you pleas explain more detail what do you mean by " are not planar but just happened to be squished flat"?

Say for instance you have an "S" shaped flat path and then you sweep a line along it - the result will be a planar surface but it's a general surface made up of a bunch of control points, it's not the most simple "analytic plane" surface which is made up of only 4 corner control points. It's generally only those analytic planes that will get this special behavior in union, not other general surfaces with a whole bunch of control points in them that just happen to have all the control points in one plane.

So for instance if you want to have that behavior you want to build things by using Construct > Planar which builds a simple plane as the underlying surface, not use sweep or network for example to build the planar surfaces.

- Michael