Synonyms: undependable, unreliable
Definition: not worthy of reliance or trust
Usage: in the early 1950s computers were large and expensive and unreliable; an undependable assistant
Similar words: temperamental, erratic
Definition: likely to perform unpredictably
Usage: erratic winds are the bane of a sailor; a temperamental motor; sometimes it would start and sometimes it wouldn't; that beautiful but temperamental instrument the flute- Osbert Lancaster
Similar words: uncertain
Definition: not consistent or dependable
Usage: an uncertain recollection of events; a gun with a rather uncertain trigger
Similar words: unsound
Definition: of e.g. advice
Synonyms: undependable, unreliable
Definition: liable to be erroneous or misleading
Usage: an undependable generalization
Similar words: fallible
Definition: likely to fail or make errors
Usage: everyone is fallible to some degree