I think one criticism is that prices would change to capture the UBI.I think I read the idea in "Progress and Poverty," although I've certainly seen it elsewhere since:

- If everyone suddenly has more money (say $2 more per day)

- And milk is a basic necessity

- The milk seller knows everyone needs milk and now has $2 more to spend

- They can gradually raise the price of milk by close to $2

- Consumers must still buy milk at the higher price

- The intended benefit of the extra $2 is effectively captured by the milk seller

The increases in general purchasing power can be absorbed by suppliers of essential goods. If you have just excess discretionary income in the general case, then non-essential goods can bump in price, too.