The point is that it makes peoples survival dependent on the whims of politicians. In the future, if they choose to disqualify you from UBI for any reason, what recourse do you have? We should not be giving politicians direct coercive control over our lives.

And to people who say "if they disqualify you, then it's not universal, ha!": legislators don't let things like simple semantics stop them from enacting laws. If they really need to, they'll just re-define the word.