I have my own problems with UBI, but as far as 'fair work sharing', the presumption would be that the work is done 99% by machines, where you don't need to be 'fair' & the remaining 1% could be highly compensated.
Consider absolute numbers. Suppose monthly food expenses drop to $100. That's 2-3 hours of work for each person to eat monthly. X 330M = 1 billion hours per month. And that's just food. In practice, fair work sharing still seems critical, unless you have a better example in mind.