To my knowledge, what you've said is just as incorrect as what the parties you oppose said. There haven't actually been that many studies, all considered, and the studies there have been weren't conclusive.

I think Finland would be better off testing the idea (e.g. in a single city, eligible population fixed at the start) before committing to it. There would doubtless be problems with the test, but it would give a better idea of what to expect.