unreliable

liable to be erroneous or misleading

• “an undependable generalization”

not to be trusted

not worthy of reliance or trust

• “in the early 1950s computers were large and expensive and unreliable”
• “an undependable assistant”

dangerously unstable and unpredictable

• “treacherous winding roads”
• “an unreliable trestle”

lacking a sense of responsibility