It is a complex issue and is a massive change to a fundamentally new type of taxation and economic system which is why some economists are unsure fo the benefits because they lack many of the necessary quantitative modelling tools.
Unworkable: no, its been shown to be workable in practice at small scales. It can be made to work at a national level but such a big change is hard to achieve. There is nothing fundamentally unworkable about it.
Expensive: Well that is kind of obvious, the real question is how much savings can it generate form simplifying current taxation, tax credits and benefits system, how an taxes be raised sufficiently to cover costs, and what benefit does it have to society. The latter point is the biggest question. What price do you put on lower crime, increased happiness, more productive workers, better social cohesion, narrower economic dive? Unemployment benefit and and child tax credits are expensive, so are state pensions and national health care. Should we do with out them due to cost?
Stopping people form working is mostly going to come down to what level the UBI is at, what existing unemployment benefits there are, salaries for actually working. Sure if you set the UBI at 30K a year, minimum wage is 3 pounds an hour and it gets taxed at 70% then no one would work and it would implode. If you set the UBI at 5K, minimum wage was 20 quid and nothing was taxed until you earned 20K a year then people would be very motivated to work. So it is important to find that balance and make the numbers balance.
I don;t think it is anything that will work short term but long term I see now way around it. With automation millions of people will lose their jobs in the next decades. Anyone from taxi drivers to doctors, programmers to factory workers will face being obsolete due to the endless march of technology. The first autonomous rucks are already on he road, first autonomous tax cars already exist and serving people. I view UBI as basically inevitable at some point.