Scientific medicine is not easy. By this point we have largely picked the low hanging fruit, and continued improvements are mostly incremental and hard won. In order to get the most out of our limited research dollars, and optimize medical practice with the safest and most effective treatments, we need to use all available scientific evidence in the proper way. That is the essence of SBM.
There are those, however, that misuse or abuse the scientific evidence — whether to promote an ideology, out of innocent ignorance, or for nefarious purposes. In order to be truly science-based a medical intervention should be plausible, or at least not implausible, based upon basic science evidence, and it should actually be safe and effective when tested in people. Therefore, medical practices can fail to be scientific for one of two broad reasons: they can be scientifically implausible, or they can lack proper clinical evidence for safety and efficacy (or even have evidence for lack of efficacy). Some modalities (like homeopathy) fail on both counts.
The more pernicious medical claims are those that seem highly plausible, that can be extrapolated from basic science, but simply lack adequate clinical evidence. Stem cell clinics are an example — they can easily dazzle desperate patients with scientific descriptions of how stem cells work, and even cite published basic-science papers showing the potential of this technology. But what they cannot do is provide clinical evidence that the specific intervention they are offering is safe and effective for the specific disease or condition they are treating.