SBM believes that rather than treating patients using evidence based medicine, you should look at the all of the scientific evidence about the efficacy of a treatment, but then weight that evidence using preexisting scientific beliefs about the world in general.
The problem is that it's not clear whose gets the decide what the existing scientific beliefs are, how the evidence should be weighted, whether this results in better patient outcomes than treating patients using evidence based medicine, etc.
+1 for pointing out the problems with SBM. While I truly believe in it, it's always good to counter some people thinking it's the ultimate solution which will somehow solve all problems in medicine.
As far as I know, the only serious proponents of SBM are the authors of the sciencebasedmedicine.org blog. There don't seem to be any scholarly articles on the subject, and the only books that exist are just compilations of the blog posts.
Wouldn't that make SBM not scientific by definition?