A lot of the UBI trials have actually had disappointing results. The arguments usually claim that it’s not a valid test because it wasn’t guaranteed for life, or the goalposts move to claim that UBI shouldn’t be about anything other than improving safety nets.
Unfortunately I think the UBI that many people imagine is a lot higher than any UBI that would be mathematically feasible. Any UBI system that provided even poverty level wages would require significant tax increases to pay for it, far beyond what you could collect from the stereotypical “just tax billionaires” ideal. Try multiplying the population of the US by poverty level annual income and you’ll see that the sum total is a huge number. In practice, anyone starting a business would probably end up paying more in taxes under a UBI scheme than they’d collect from the UBI payments.