A systematic review like this can be helpful, in that it identifies where there are gaps in the literature, and prevents hype - if some studies show evidence of effect, and others do not, even if there are only a few published studies then we know somthing new about the totality of the literature on the subject.

That said, this particular systematic review has a couple of issues (e.g. I can't find the precise inclusion / exclusion criteria, nor can I find that it has been pre-registered on Prospero or another database).

I have written a few systematic reviews where there is very little data already availabe, and we use them to explain to funders why we need to do further research on a given topic.