Look out for unpublished results of fair comparisons. All results of studies should be reported otherwise estimates of effect of interventions may be biased.
Many studies never get published, and some effect measures are sometimes left out. Those that do get published are more likely to report large effects. As a consequence, reliance on published reports sometimes results in the effects of interventions being overestimated. Biased under-reporting of research is a major problem that is far from being solved. It is scientific and ethical malpractice and wastes research resources.
REMEMBER: Be aware of the possibility of biased underreporting of studies, and whether or not the authors of reviews (systematic or not) have addressed this risk.