doubtful vs unknown quantity

doubtful

• A doubtful person or thing. 

• Experiencing or showing doubt, sceptical. 

• Subject to, or causing doubt. 

• Improbable or unlikely. 

• Unclear or unreliable. 

• Suspicious, or of dubious character. 

• Undecided or of uncertain outcome. 

unknown quantity

• A person or thing whose nature or value is a mystery. 

• Used other than figuratively or idiomatically: see unknown, quantity.