I think to some extents not just advances in 3D printing, but in the quality of metal plating on polymers is allowing for huge jumps in what is possible. Huber&Suhner have a process that involves molding thermopolymers to high precision with lots of cavities/slots/etc and then using a propriatary process to metal plate them to build very high performance millimeter-wave structures like waveguides for cents instead of hunderds of dollars. Already used in mass production for automotive radar.