Here's what rhymes with underpowered.

Consider these alternatives

ineffectual / intellectual obsolescent / present unsuited / routed unreliable / liable pleasingly / increasingly imprecise / price impractical / practical outmoded / noted pathetically / theoretically ungainly / mainly feeble / people inoffensive / extensive unsuitable / suitable undermanned / hand grossly / closely unattractive / attractive unsophisticated / dated homely / only