I get the feeling that a lot of people say that it's unworkable/unaffordable based on no evidence other than, perhaps, some intuition about how things *should* work. But it's vital to test those intuitions in practice, because many end up being some degree of wrong.
It's true that the initial cost projected over the whole country would be large, with the exact cost depending somewhat on how you target it, but it's not true that you get none of this money back. For example, when you give less well-off people money, they tend to spend it -- on food, holidays, basic healthcare, technology items etc. That means that the money is, mostly, doing something useful. In that sense, this can be equally seen as a large investment, and you could expect in the long run to make much of this back and then some. And, besides, you have to compare the cost to the present system, which itself is expensive, and it's not difficult to find claims that in fact you'd make savings in the long run compared to that.
Anyway, the exact cost depends on how you organise the scheme (obviously), so writing it off as unaffordable without seeing the details *or* without any trial seems premature.
A few more links:
https://ralphbuncheinstitute.org/2021/03/where-is-the-ubi-movement-at-right-now-with-karl-widerquist/
https://web.archive.org/web/20220105225751/https://www.zeit.de/zustimmung?url=https%3A%2F%2Fwww.zeit.de%2Fwirtschaft%2F2017-02%2Fthomas-straubhaar-buch-bedingungsloses-grundeinkommen-auszug (in German)