It is possible that there would be fraud around pretending to be a person when that person doesn’t exist, or continuing to collect benefits for someone who is dead.

But “are you a live person” is much harder to fake than “did you make sure you didn’t do these 6 illegible things that would make you ineligible”.

When you eliminate means-testing in favor of income taxes and eliminste the myriad different definitions of income and assets for qualification for different programs and the opportunity it creates (and the incentive funding cliffs create) for specialized benefit fraud, there is no special “UBI abuse” to monitor for. You have to monitor for tax fraud, but we’ve got a bureaucracy doing that at society-wide scale anyway.