I'm VERY curious why you mention swain, hyfe. From experience his cds are too long to outdo nasus' sustain. Any time nasus lands wither + SF + q on swain, he comes out on top of the exchange. Admittedly i've only played the swain matchups in normals, but nothing about swain's kit stands out to me as a counter to nasus, not until swain gets ult and by then if nasus has free farmed its too late.
(one game i literally had 100 cs more than swain in laning phase, 36 to 136~, but that was a normal so probably doesnt count)