There are two versions of the proposal. UBI as a replacement for social programs is fantasy. The UBI cheque would have to be insanely fat to replace the welfare/medicare/housing/food all the other forms of assistance that disadvantaged people need. I know a lot of libertarians support it because it is essentially a round about way of cutting social benefits.

And hypothetically speaking, say the government is spending $10k/year on assistance to a disabled individual. If you instead give $10k/year to that individual in cash he would not be able to buy the same level of care for the same price on the private market. He'd get massively short changed. Welfare is far more efficient, financially speaking, than handing out cash to people and letting them fend for themselves.