Or search: stiff, sturdy, adamant, ossified, inelastic, unbending, semirigid, fossilized, inexorable, adamantine, brassbound, fossilised, unadaptable, intransigent, uncompromising
3 Syllables: SUU
credible, edible, equable, flexible, medical, sectional, skeptical, spectacle
4 Syllables: USUU
acceptable, conjectural, delectable, detectable, electoral, exceptional, execrable, impeccable, impregnable, incredible, perceptible, perceptual, receptacle, regrettable, respectable, susceptible
5 Syllables: UUSUU
architectural, directional, hypothetical, unacceptable, unforgettable

Found 29 words across 3 groupings. Skipped 1 excess word of a similar stem.