Or search: unpassable, unclimbable, untraversable, unsurmountable
2 Syllables: SU
national, natural, rational
3 Syllables: SUU
affable, animal, arable, cannibal, capitals, casual, comparable, fallible, fashionable, lateral, laughable, mackerel, marital, Nationale, passable, pastoral
4 Syllables: USUU
bilateral, collateral, compatible, diagonal, grammatical, hexagonal, implacable, infallible, inflammable, irrational, octagonal, pyramidal, unnatural
5 Syllables: UUSUU
incompatible, multinational, preternatural, supernatural, unilateral

Found 37 words across 4 groupings. Skipped 4 excess words of a similar stem.