unsound vs mentally ill

unsound and mentally ill both are adjectives.

Word	Noun	Adjective	Verb	Adverb
unsound	No	Yes	No	No
mentally ill	No	Yes	No	No

As adjectives, unsound and mentally ill are synonyms defined as:

Other synonyms of unsound include unstable.

unsound (adjective)	mentally ill (adjective)
not sound financially	suffering from severe mental illness
not in good condition; damaged or decayed
suffering from severe mental illness
containing or based on a fallacy
physically unsound or diseased
of e.g. advice