Union btw objecrs extruded from spline and lines

applying complex relief features to a flat surface. I’m joining the complex objects to the planar surface with “union”. The issue becomes apparent after the union is executed in the following way: the smooth curves of the complex objects I’ve extruded from spline sketches suddenly become facets. The base objects I’ve I’m creating the union with is extruded from a rectangle. In some cases, the resulting union even has the vertical walls “peeling” off from the curve. Is there a simple fix for this?