doubtful, dubitable, questionable, unsure, incertain, uncertain, unconvinced, in question, jubious, dubitative, more...

certain, clear, definite, evident, indubitable, plain, sure, unambiguous

highly doubtful,   more skeptical,   is iffy regarding,   more doubtful,   don't doubt,   slightly suspicious,   less sure,   very questionable,   questionable morals,   very skeptical