harshness has 14 distinct synonyms spread across 4 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the roughness of a substance that causes abrasions
the quality of being harsh or rough or grating to the senses
the quality of being cruel and causing tension or annoyance
Synonyms, antonyms and the broader and narrower relations for harshness come from Princeton University’s WordNet. Full sources and licences.