incompetent has 8 distinct synonyms spread across 5 of its 6 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 competent.
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.
legally not qualified or sufficient
“a wife is usually considered unqualified to testify against her husband”
not qualified or suited for a purpose
“an incompetent secret service”
someone who is not competent to take effective action
Synonyms, antonyms and the broader and narrower relations for incompetent come from Princeton University’s WordNet. Full sources and licences.