21% are long term homeless. And that's what people generally mean when they collegially use the term homeless. That's defined as homeless that has lasted for more than a year and has significant social or health component, such as substance abuse or mental illness. The thing about mental illness is that there is also treatment for them. The solution is different for them as for the substance abusers.
If you ever see a Finn begging it's pretty much always an alcoholic who wants more beer. That's because the mentally ill cannot really beg as they're either receiving treatment or if they have unfortunately slipped trough the cracks of the system they're also unable to beg for any extended period.
As for the scale of this problem. 2019 there were 961 persons listed in the long term homeless category. That's out of a population of 5.518 million in 2019. That's 0.017% of the whole population. Also, even out of them 584 live with someone, just not with an official address. They wouldn't be counted as homeless in US.
The actual amount of people in street on that category was 177. That's 0.003% of the population.
Check out page 845 showing death by alcohol poisoning is 5x higher in the homeless population of Finland than the whole of the country.
FWIW, and anecdotally, my ancestry is Finnish and we are known for having issues with alcohol. The majority of my extended family has drinking issues. Within our Finnish-American community, it’s believed that Finns have some sort of gene that pre-disposes us to drink to excess.
You do understand that everyone gets social security as money if they need. The problems happen if they are unable to use that money to pay the rent. And yes indeed the main reason is some sort of intoxicant use, as they rather get more stuff than use the money for rent.
Legal debts are also not a reason not to pay rent, as one is protected from repaying them when it's about essentials. So that doesn't count either. As long as you're able to push the pay button in your online bank you use the default system. Only when that's not possible do you fall into the provided housing system. And not surprisingly drug use is a major reason for not pushing that pay button but rather taking the money and using it elsewhere. Does that honestly surprise you? What else could it even be?
That's why we have the second option with food stamps and provided housing. It's not perfect as people elsewhere have stated the obvious "Hey want to buy 20e foodstamp for 10e?". But still they get it.