It has been my experience that copper is not resilient enough to conform and continue to exert enough force over the microscopic imperfections that appear when the engine is up to operating temperature and the distortion that occurs due to combustion pressure on each power stroke. A head gasket has to be able to flex and conform to these moving surfaces over a hundred times per second to provide 100% sealing.
The quality of the sealant is what is largely responsible for the sealing action when a copper head gasket is used. I do not recommend doing this ( has been done when doing dyno testing) but the copper gasket is basically a spacer and similar sealing results can be obtained using a good sealant between the head and cylinder providing the head was machine to provide the correct squish clearance and volume.
A rubber o-ring, conventional composition type head gaskets or stainless steel gasket that has embossed ribs do a much better job of sealing of these moving sealing surfaces.