Or search: grim, stern, merciless, inexorable, relentless, unmerciful, unforgiving, unrelenting, unmitigable, unappeasable
2 Syllables: SU
national, natural, rational
3 Syllables: SUU
admirable, affable, animal, arable, cannibal, capitals, casual, comparable, fallible, lateral, laughable, mackerel, marital, Nationale, passable, pastoral, taxable, tractable
4 Syllables: USUU
adaptable, bilateral, collateral, compatible, diagonal, grammatical, hexagonal, impassable, impractical, infallible, inflammable, intractable, irrational, octagonal, pyramidal, unnatural
5 Syllables: UUSUU
incompatible, multinational, preternatural, supernatural, unilateral

Found 42 words across 4 groupings. Skipped 4 excess words of a similar stem.