it seems that hugin/nona doesn’t do any lowpass filtering when interpolating images, leading into aliasing. this can be rather annoying when mapping equirectangular imago to stereographic (eg. little planet images) projection. apparently panotools supports[1] wide variety of antialiasing filters, but none of them seem to be used by hugin?! 
moreover, sinc1024 filter seems to suffer from pronounced ringing, but that seems to be by design.
another thing I noticed that the nona-gpu output isn’t identical to cpu nona output. the white line is only present in the gpu output.
1: http://photocreations.ca/panotools/stitch.txt
the test images were simple equirectangular images filled with alternating black and white lines with pitch -90 degrees, remapped into 90 degree rectilinear images at the optimal pixel scale as calculated by hugin. this simulates extracting of equirectangular panorama’s nadir or zenith region into its own image.