Or search: stiff, unwavering, unbendable, unassailable, incontestable, incontestible
3 Syllables: SUU
amiable, capable, changeable, favorable, payable, traceable
4 Syllables: USUU
attainable, available, debatable, incapable, insatiable, obtainable, occasional, relational, rotational, sensational, unchangeable, vocational
5 Syllables: UUSUU
ascertainable, computational, Congregational, conversational, educational, gravitational, inescapable, inspirational, interchangeable, observational, occupational, operational, recreational, situational, unassailable, unattainable, unavailable, unmistakable
6 Syllables: UUUSUU
denominational, organizational

Found 38 words across 4 groupings.