Synonyms of unsubstantiated

2 Syllables
dubious, iffy, doubtful, fishy, suspect
3 Syllables
theoretic, baseless, arguable, unfounded, suspicious, uncertain
4 Syllables
apocryphal, controversial, conceptive, debatable, ideational, disputable, dubitable, unconfirmed, theoretical, foundationless, unsupported
5 Syllables
speculative, formularized, indefinite
6 Syllables
uncorroborated, unauthenticated