Systematic reviews can only contain resources and references that have been published. However, as any writer of reviews knows, searching for the latest research on a topic can be problematic. We need to be aware of what we aren't seeing as well as what we are seeing. How do you know you are getting the latest and greatest information? Or perhaps you are running into duplicate publications (the same data being re-hashed over and over in different sources). The biggest problem you will likely face when searching however, is that there may not be data, not because the data doesn't exist, but because it was never published. It is likely that people have asked the same questions that you are asking and maybe even evaluated it as part of a formal study, but if the results weren't novel or the intervention failed, the work may not have been published.
Studies have been conducted to determine just how many negative studies and trials go unpublished. The general consensus is that about one-third of biomedical and medical studies go unpublished because the results are negative. That is, the hypothesis was not proven and the experiment failed to be "interesting." When it comes to drug trials, the estimate of unpublished negative studies is higher; as many as 50% may go unpublished. This makes performing a systematic review even more vital in gathering relevant and valid evidence. We need to be aware that there is often a positive bias to literature searches; which is to say that it is more likely that you will find an article showing a positive outcome with an intervention than a negative or insignificant outcome. One just needs to be aware of this fact when assessing the information.