Tax revenue isn't the only measure of success. If people take UBI, then stay home to raise their kids or support a sick relative, that work has a value to society that isn't accounted for -- savings in spending on old age homes for parents of people on UBI were likely missed or under-represented. Someone who stays home, but volunteers instead provides a service to society at large whose benefit is difficult to quantify when the impact may not be strictly monetary, or whose monetary impact may be delayed for several years.

The other side is that UBI means workers aren't exploited by making housing contingent on working... Employers will likely need to pay more and offer better benefits (that affect life/work balance) in order to find staff... Another intangible/incalculable improvement for society that doesn't show up in a small-scale study.