UBI is a great idea in theory. But so is giving everyone everything for free. Forever.
And it is the "forever" bit that is the problem. All the descriptions of UBI portray it as something that will go on for a year or two .... maybe 5 years, tops.
But consider 100 years in the future. Where will the UBI come from then? What will the citizens of 2117 be getting. And more importantly, what about the families that have known nothing except getting all their money from the state for as long as any of them can remember - what will they be like?
Will they have any education (unnecessary when the UBI pays for everything, and expensive). Will they have any skills? If UBI stopped - what would they do. And most important of all: when your entire population is dependent to a greater or lesser extent on government pay-outs, do you still have any form of democratic process? Or is the entire place run by a small elite who can use money to keep the vast majority in subservience: too scared speak out, or do anything wrong in case their UBI gets cut.