The main issues I have with a flat UBI is that the amount won't be set high enough to aid the most disadvantaged, especially if UBI is used as a replacement for traditional "welfare" & disability support programs; and that it will never fly here in the US due to the evangelism of the "work ethic" & the myopia of having to prove your suffering to receive relief conditioned on "payback".
A Progressive UBI that gives more to those at the bottom is much better.