unwieldiness has 2 distinct synonyms spread across 1 of its 2 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 quality of being difficult to direct or control by reason of complexity
“avoiding the unwieldiness of formal legal processes”
trouble in carrying or managing caused by bulk or shape
“the movers cursed the unwieldiness of the big piano”
Synonyms, antonyms and the broader and narrower relations for unwieldiness come from Princeton University’s WordNet. Full sources and licences.