Or search: grim, stern, adamant, implacable, inflexible, relentless, adamantine, unforgiving, unrelenting, intransigent, unappeasable
3 Syllables: SUU
admirable, favorable, miserable, preferable
4 Syllables: SUUU
answerable, favourable, honorable, honourable, measurable, memorable, pleasurable, reputable, separable, tolerable, venerable, vulnerable
4 Syllables: USUU
desirable, discoverable
5 Syllables: USUUU
considerable, dishonorable, dishonourable, disreputable, incomparable, innumerable, inseparable, insufferable, insuperable, intolerable, invulnerable, irreparable, objectionable, recoverable, unalterable, unanswerable, unconquerable, unfavorable, unfavourable, unutterable
5 Syllables: UUSUU
undesirable
6 Syllables: UUSUUU
inconsiderable, unendurable, unexceptionable

Found 42 words across 6 groupings.