The problem with all of these trials is that they're both limited in time and scope. UBI is a complete overhaul of how the economy works and you can't do a limited trial to find out the full effects of that.
In general, a UBI trial typically involves selecting X number of participants (random, based on age or where they live), giving them $Y per month for a period of Z months and then measuring things like labor participation, health outcomes, subjective happiness ratings, etc...
These trials have positive results, because as it turns out, if you give unconditional money to people, they tend to like it. This isn't really news.
What the trials fail to investigate is the funding side of such a scheme. The money for the trial comes from a one time expense in the government budget or from a non-profit organization. In a nation with UBI, the tax system has to be reworked to fund the UBI. This would typically entail major changes to how the tax system works, which can have profound impact (good or bad) on the economy. This part isn't trialed, and it would be very hard to do so.
In addition, one of the benefits of UBI is that people would be free to explore other ventures beyond just working for a salary. A concern that critics have is that this would greatly decrease participation in the labor force. Trials have shown this decrease to be much smaller than expected, which is touted as good news.
However, people go into such trials with a very different mindset than if UBI would actually be introduced. If you're receiving your UBI during a trial period that you know will come to an end relatively soon (in say 1-3 years), you'll be far less inclined to give up a job that you're OK with than if UBI were to be a permanent thing. Because once the trial ends, you'll have to find a new source of income again.
The famous Mincome experiment saw the largest decrease in labor participation among recently graduated students and new mothers. And that perfectly aligns with what I described in the paragraph above: The biggest effect is seen in groups that can afford (career-wise) to spend some time outside the labor force. Recent graduates haven't started their career yet, so starting it a year or two later isn't a big deal. And new mothers were already expected to spend some time away from work (note that Mincome happened in the '70s, when women staying home with young kids was still the norm), so adding a year or two to that again isn't a big change.
I'm not opposed to UBI. But I'm highly skeptical about the trials that are being done to demonstrate its benefits. These trials are too limited to paint a fair picture of the benefits and risks.