Even with systematic reviews, which are the highest form of scientific evidence, there is still a lot of room for subjectivity. You can develop a systematic review in a way that makes it more or less likely that you will find a certain outcome, just as you could with an individual study. Not only that, but the review depends on the objectivity of the people screening articles, who could (intentionally or accidentally) systematically include or exclude articles that may have an impact on the review’s ultimate conclusions. And then of course the authors have to synthesize data and come to conclusions, both of which are mostly subjective activities.
That is from this very interesting post. (Source: "The Browser").