inadequacy has 3 distinct synonyms spread across 3 of its 3 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. Its one recorded opposite is adequacy.
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.
lack of an adequate quantity or number
“the inadequacy of unemployment benefits”
a lack of competence
“pointed out the insufficiencies in my report”
unsatisfactoriness by virtue of being inadequate
Synonyms, antonyms and the broader and narrower relations for inadequacy come from Princeton University’s WordNet. Full sources and licences.