A systematic review should aim to search for and be inclusive of all relevant studies. Problems occur when studies are missed or ignored and this can limit the systematic review’s validity.
A systematic review uses appropriate methods to ensure it is methodologically sound. Errors in the approach to conducting the review or lack of expertise can jeopardise the internal validity of the systematic review.
A systematic review should report it’s methods with enough detail to enable replication. If the reporting quality of a review is poor or the full methods are inaccessible then the systematic review is not reproducible.
A systematic review should be conducted fairly, by teams with appropriate expertise. If the review is done in a perfunctory way by people who do not care about the conclusion or by people with vested interests, then the systematic review may not be reliable.