I don't trust UBI because I don't trust the average person to manage their free time and money carefully. This just feels like a road to boredom, followed by doing stupid shit, including drugs.
Anyone who have taken a month+ old vacation probably knows what I'm talking about. There comes a point where you just (unintentionally) start degrading yourself because there is no purpose in your life, whether by playing videogames all day or substance abuse
It's even worse when you realize most homeless have psychological or physical problems that made them homeless in the first place, and throwing money at them is likely not going to extend their lives if you know what I meant, or fix said problems.