They don't criticize it, but believe UBI will "almost pay for itself" by not requiring aa much overhead. Which it won't, not even remotely close.
Many problems come from an increasing lack of purpose in society. Getting paid to do nothing will not solve that for probably 99% of the population. Lots of idle time for lots of bored people is like pouring gasoline on a fire.
Mechanically the other problem would seem to be, if you listen to someone like Gary Stevenson, that it only works if you ratchet up taxes on the top end.
Otherwise broad flat cash distribution from the government generally causes inflation and all the money ends up workings its way up to the wealthier. So if you do not tax it back, it actually ends up being regressive.
The mechanism is something like - the poorer you are, the higher % of your income, by necessity goes to spending on basic needs. You have a zero or negative savings rate. The richer you are, the opposite. You have savings you put into income producing assets (stocks which are fractional ownership in companies, real estate, etc).
So if everyone gets $25k/year, the bottom end will spend it all on goods & services (food, clothing, rent) that are owned/produced by the wealthy. And it compounds as the wealthier then are able to buy more and more income producing assets from the middle class.
That’s not what I'd call a problem (its part of most concrete UBI proposals), but, yes, whether you look at it through a classic fiscal lens or a macroeconomic impact lens, you have to raise taxes concurrently if the UBI is significantly greater in aggregate payments than the means tested welfare it replaced (which it must be to maintain the same base benefit level, and many proposals would increase the base benefit level), and any sensible implementation will do it progressively starting somewhwere above the middle of the income distribution.
Politicians need to win elections every 2-6 years, and honestly most of them aren't that bright.
Explaining to middle class people that they are going to get $20K UBI but their taxes are going up $18K isn't going to go well.
Remember whenever you setup a "good" government program thats dependent on 1-2 other "bad" government programs in unison (UBI + progressive tax increases) then the risk is future admins remove the medicine but keep the candy. Then the whole thing becomes unaffordable and the good program gets wound down.
Or you end up with crazy stuff like the UK triple lock pensions.
Two mitigations would be gradual adjustments, and a willingness to delay reductions a bit.
People shouldn't be sweating bullets about help being pulled prematurely as a direct result of trying to get past the need for it. Or have the marginal impact of increasing their earned income actually reduce total help+income.
I know somebody in an extremely bad health situation, and dealing with both of those perverse issues. Attempting employment would carry a lot risk. And with kids to be cared for, playing roulette in an already challenging situation is a real barrier. (In this case, it isn't government help, but a situation with similar logic.)