It is very high. They have a ton of citations in the page for every supplement. For each health claim for each supplement they list an effect size and a level of evidence based on the citations. They call this the human effect matrix and an example for fish oil is here. Not all the pages are as extensive as fish oil, obviously, because it is one of the most-studied nutritional supplements available.

Edit: I see now that you are probably referring to their "Reference Guide" rather than the site itself. I have never read it, I was only commenting on the information available on the site.