One study investigated the quality of effectiveness reviews in health promotion [1].  It found that existing reviews are often poorly reported, and in reviews addressing similar topics, the use of different methods can lead to different review conclusions. Less exhaustive search strategies find fewer studies for reviews. Varying the inclusion criteria in a review can result in different recommendations for what constitutes an effective/ineffective intervention. 

Commissioners, purchasers and providers of health promotion services had clear views about how effectiveness reviews could be made more useful. In particular, they highlighted the need to examine intervention processes as well as outcomes and involve end-users in framing the questions and scope of the review.

The study makes detailed recommendations for the reporting and preparation of systematic reviews.

Findings from a series of workshops on systematic reviewing can be found here.

References

1. Effectiveness reviews in health promotion (1999)