The UBI you want must be indexed to (real) GDP per capita. This way inflation cannot erode it, working-class power increases over time, and there's no disinvestment issue related to a reduced incentive to work. As profitability would decline, the way out of stagnation would naturally be worker cooperatives and other non-profit activity.
In contrast, job guarantee (as the USSR had) is either a bureaucratic mess or a productivity disaster, and most likely both.
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Properly indexing UBI mechanically implies shorter hours down the road, unless productivity stagnates or the need to consume increases. Either condition is not so easy to invoke.