I think the employees would be forced to increase their salaries to a significant amount above the UBI in order to attract people who would be willing to perform the work. That in turn, would lead to massive inflation, which in turn, would turn the UBI into something that won't cover the basics.
That in turn, leads to increasing the UBI, which causes the companies to have to increase their salaries, which again, results in massive cost increases for consumers, and on and on it goes, until the currency has lost all trust and collapses.