Where other than Finland is it being tried? What is systematically failing is to expect people to get clean before providing housing. It doesn't get people clean and it doesn't get people off the streets.
See my response to your comment below. It's the general case (where I live, at least) that people aren't required to be clean to get housing. But they are required to be part of a rehab program is a therapist/counselor/case worker has deemed addiction to be a contributing factor to their circumstances. They don't have to be clean in those cases, but working toward being cured of their addiction.
