WordNet sense 1 (containing or based on a fallacy):

fallacious

WordNet sense 2 (having no cogency or legal force):

invalid, invalid

WordNet sense 3 (damaged by decay; hence unsound and useless):

decayed

WordNet sense 4 (not financially safe or secure):

bad, risky, speculative, high-risk

WordNet sense 5 (suffering from severe mental illness):

unstable, mentally ill

WordNet sense 6 (afflicted with or characteristic of mental derangement):

insane

WordNet sense 7 (physically unsound or diseased):

unfit

WordNet sense 8 (not in or exhibiting good health in body or mind):

unhealthy

WordNet sense 9 (not worthy of reliance or trust):

undependable, unreliable