Who could possibly be against "evidence-based" policy?
Some things are obviously and self-evidently stupid. Socialism for example - high marginal tax-rates, nationalisation, closing down markets where possible in favour of state monopolies failed. And in as perfect an economic experiment as any undertaken, two nations, both shattered by war and populated by Germans went head to head. The Capitalist system turned out to be much, much less shit than socialism. Yet many social "scientists" still seem intent on manufacturing evidence that the solutions once tried in East Germany are not only feasible, but that any other approach is both doomed to failure and wicked.
Instead of evidence-based policy, what you often get is policy-based "evidence". You have the same political arguments, dressed up in a kind of pseudo scientific hocus-pocus.
Take the "debate" about minimum pricing as a classic example.
First make a heroic assumption. Assume a fall in alcohol consumption per head is desirable (it isn't, what we want to do is reduce "problem" drinking). Second, ignore the fact that your desired outcome is happening anyway. Third, ignore all the evidence that "problem" drug-takers have a lower elasticity of demand and assume that minimum pricing will mostly affect the consumption by alcoholics. Fourth, express these assumptions in a spreadsheet, with no real-world evidence. Fifth, describe this spreadsheet as a "model". The zeroth step is, of course to get a university to describe you as "professor" first. Then you're able to tout your guesswork and call it "evidence", to politicians, and unmolested by any critical thought on the Today program and be paid handsomely from tax-payers' funds to make this "evidence" up into the bargain.
So you have an "evidence-based" policy to impose a minimum unit price on Alcohol. It's regressive, and probably won't work. It will reduce moderate drinking by sensible people, making them at the margin, unhappier. It is unlikely to reduce problem drinking, but may make problem drinkers substitute clothes, or food, or heating for their more expensive booze. Nice one. Everyone's poorer.
The same is true with social services' interventions in family. You can point to the number of successful interventions, but there's no-one measuring the opportunity cost of responsibility not taken, or families broken up unnecessarily. Or regulation in Banking - it's impossible to deliver a counter-factual, and everyone's trying to defend their decisions.
Or climate-change. Whilst I'm almost convinced the climate's changing, and we're responsible, what's preventing me being ACTUALLY convinced by the evidence for Anthropogenic climate change caused by C02 etc... is that no-one's funding research into any other hypotheses. All research grants flow through councils who're totally committed to a single theory. The lack of understanding of feedback loops, and the total lack of any predictive power of the models suggests our understanding of a chaotic system like the climate is limited. We're probably on the right lines, but anyone who thinks otherwise is effectively shut out of funding. Therefore the shriller the POLITICAL consensus for wind-farms (for example) the less convinced I am by the SCIENTIFIC consensus. The obvious nonsenses from both sides (look at the weather - there's climate change flooding your house/Ha! climate scientists stuck in the ice) means this is becoming less about science, and more about political articles of faith. There's been too much policy-based evidence-making based more on distaste/support for big business, than any climate scientists' actual views. And what does a climate scientist know about the economics of electricity generation anyway?
You can go through almost any area where government claims to be "evidence-based". The evidence given to politicians is nearly always policy-based. This is why politicians make crap decisions, and they'd be better off just leaving us alone.