Or search: indubitability, indisputability, unquestionability, unquestionableness
2 Syllables: SU
Nicholas
3 Syllables: SUU
bitterest, differences, distances, frivolous, limitless, rigorous, Spiritus, syllabus, timorous, vigorous, villainous
4 Syllables: USUU
antithesis, carnivorous, felicitous, indigenous, solicitous, vociferous
5 Syllables: UUSUU
abolitionist

Found 19 words across 4 groupings. Skipped 1 excess word of a similar stem.