To be considered homeless, you just need to be without a permanent place to live. Some people are living in their car and still employed, some are couch-surfing, some are sleeping on the sidewalk and have severe drug/mental health issues.

Housing first/financial aid is great for the first two people I mentioned, it's not too helpful for the third. People often look at trials like this and think it's an easy solution to homelessness while ignoring the problem just isn't that simple because of that third group.

All that said, if the program does a simple evaluation to determine which group people fall into and gives money/housing to those best suited for it then it's pretty much a no-brainer that it should be widely implemented. It won't solve homelessness, but it'll make a really big dent.