Using a polarizer with ultra-wide lenses does cause another issue. As maximum polarization occurs at 90 degrees to the axis of the light source you may notice one side of the image much more polarized than the other side, particularly in skies. Or the center of the image, assuming you are pointed at a 90degree angle to the light source/sun, is very blue while the edges on either side are much more pale(un-polarized). The tools in post can be better at darkening a sky, when using wide lenses.
also the curvature of the edge of the wide angle lens verses the flat surfaces of the filter, can also result in some loss of sharpness near the edges.
Just an observation...