Assume everyone needs $100 a month for room and board.
Assume a free universal basic income of $100 is implemented.
On the margin, someone will stop working. There will be less wealth. With less stuff bidding for a fixed dollar amount, the price of money will fall. Inflation occurs. Room and board now cost at least $101, and unemployed UBI recipients are now either getting evicted or starving. Notably this only occurs if UBI is national. Having a limited test run won't shift the inflation needle beyond the noise.
Right, so let's start a little higher, yeah? Start at $120. On the margin, someone will stop working. This will cause inflation. Meaning $120 will only buy, say, $115 worth of stuff. This widens the indifference margin between working and not-working, so someone else will stop working. This will cause inflation. Which will widen the not-working margin. Und so weiter.
(Indeed subsequent rounds of inflation are all but guaranteed to be higher, as a roughly equal mass of stuff is being removed from a smaller pie.)
I've mentioned serfdom before. If the guaranteed income came with some kind of cost, being universal only in availability rather than in concrete application, the damage would in all likelihood be negligible.