There's an excellent take-down of the "evidence-based policy" movement at SciDevNet. Author Erik Millstone seems to have a pretty firm grasp of things:
...the relevance of...models is more often assumed than it is demonstrated. In the case of climate change, some computer models of the impact of greenhouse gases on climate might usefully approximate to global realities.
Science advisers often ignore or conceal key uncertainties when offering judgements, perhaps catering to policymakers’ preference for reassuring oversimplifications
...some stakeholders might claim a uniquely authoritative understanding of an issue based on evidence