I think the UBI experiments were one-sided. They may have proven that the receivers have profited. But did they show that the givers profited as well, or at least did not suffer enough to significantly reduce their economic output and therefore risk the financing of UBI?

You'd need to check something like take a group of high-earners, increase their tax rate by say 100%, increase their cost of living, and validate that they neither leave the experiment nor work significantly less.