The problem is that the imperfections are systematic and the same imperfections inflict every experiment.

To use an extreme, let's say you just up and gave someone 10 million dollars. Would they do pretty well? Probably would live it up pretty well or die trying. Assuming they avoided vices that would kill them, they would probably have whatever sort of house they want, any sort of car they want, and so on. They might engage in some noble occupation now that they don't care about money that sounds nicer than most jobs to make money. You could repeat this same experiment dozens and dozens of time and come away with the same conclusion, that giving people money without conditions improves their situation. At 10 million, you probably wouldn't see the same data about 'gainful employment' as you do with $5,000 dollar experiments, where they know that won't get them going, but other facets should reproduce.

What if you gave 340 million people 10 million dollars? Well, the economy would adjust to deal with that new normal. You would have massive inflation to compensate for the glut of cash. The market for Ferrari's jumps a thousand-fold, but they aren't going to be making a thousand-fold more cars, they'll just price them up to the point where even millionaires can't afford them anymore.

What if you gave 340 million people 4 dollars a month? Nothing would happen, it's too meager to move the needle.

So a UBI would be somewhere in the middle. Now the question is whether there is a sweet spot with the right impact. Economically, it's likely that their opportunities in 'objective' terms may be about the same before and after, but they might feel less crappy about it. Though over long enough time they might get pissed that their UBI payment isn't going to be even sustenance income even if it started that way.

The other common trend in these is some romanticism of some jobs over others. He was stuck mowing public parks, but with the UBI experiment, he was able to better himself and get a more respectable job. Good for him, but who is now mowing those parks? Oh someone who wasn't in the UBI experiment. So if everyone has UBI, you can't have that nice story for everyone, someone is still going to be doing that "lame" mowing job.

UBI might be a part of dealing with a hypothetical labor surplus when we just don't know what people should be doing, but it's going to be rough. If it's too low, you've doomed people relying on UBI to forever be stuck with crappy standard of living because there would be no jobs for some of them to have the chance to supplement the income. If it's too high, then why would anyone ever mow that public park?

I suspect shorter and shorter work weeks has to be a mandated solution, rather than some fixed money amount injected per capita. Spread those crap jobs over more people. Instead of unrelenting 40 hours of landscaping a week, a couple hours a week and surplus labor means you get 20-fold more people doing the job. Though first we have to actually have an obvious labor surplus to get that going.