While I'd say it's generally been successful, the experiments with housing first programs here in Canada have failed some specific categories of people pretty badly.

In Calgary and London, people with serious mental illness in particular would get an apartment, only to end up facing eviction before long because of conflicts with neighbours or property damage. Some were housed for less than a month before eviction.

Someone with active psychosis, someone with substantial market income but fleeing an abusive partner, and someone with a stable social and personal situation but who simply can't afford rent on their fixed pension, all require different interventions to keep them stably housed in the long run.

"Housing first" has a habit of being "housing only" support, which works for many people but not for everyone.