【Doubtful】 , 【dubious】 , 【problematic】 , 【questionable】 are comparable when they mean not affording assurance of the worth, soundness, success, or certainty of something or someone.

【Doubtful】 and 【dubious】 are sometimes used with little distinction.

【Doubtful】 , however, is commonly so positive in its implication as almost to impute worthlessness, unsoundness, failure, or uncertainty to the thing in question.

【Dubious】 stresses suspicion, mistrust, or hesitation (as in accepting, believing, following, or choosing); thus, a man of 【doubtful】 repute is by implication more distrusted than one of 【dubious】  repute; one who is 【doubtful】 of the outcome of a project has by implication better grounds for fearing its failure than one 【dubious】 about it, for the latter may imply mere vague suspicions and fears and little evidence.

【Problematic】 is the only one of the terms here considered that is free from a suggestion of a moral judgment or suspicion; it is especially applicable to things whose existence, meaning, fulfillment, or realization is very uncertain, sometimes so uncertain that the probabilities of truth and of falsehood or of success and of failure are nearly equal.

【Questionable】 may imply little more than the existence of doubt respecting the thing so qualified.

It more commonly suggests doubt about propriety and may imply well-grounded suspicions (as of immorality, crudity, or dishonesty) that for one reason or another need to be expressed in guarded terms; thus, to say that a man is a 【questionable】 character is to cast a reflection on his honesty or morality; 【questionable】  dealings suggest underhandedness and dishonesty.