UBI as a macroeconomic lever to pull is an intriguing idea - hadn't thought of that consequence. Although it figures changing the amounts would get extremely political.
The government grants a job to anyone that wants to work at a minimum wage salary. When unemployment grow, the government automatically expend more in the economy because more people access to the job guarantee.
When the economy recovers the private sector take workers away from the job guarantee through paying a little more than the minimum wage (or the same and better conditions) and the government spending is reduced.
It's both an inflation and an unemployment stabilizer. UBI doesn't have those properties.
The guaranteed job need not be pleasant. Hell, many already aren't.
But a job would be guaranteed and would meet basic needs.
But the original question was how to provide incentive. Assuming able-bodied persons must provide some work, opportunities for advancement, more pay, or more favourable environment, and the prospect of less attractive work, remain. Military and similar environments provide models, e.g., latrine duty.
Most people would prefer some useful occupation, I suspect, however humble.
And again, with guaranteed employment, even the worst job would provide a livelihood.
There are different ideas, but, I suppose that you could see it like a training program too.
Also, there are some supposed psychological benefits to work. And there is always something to do at the local community level, so it would not be totally unproductive work.
Also I don't think it's necessary that a job guarantee make an employee immune from being fired. I'd imagine a job guarantee would have all kinds of jobs ranging from relatively simple (eg. cleaning) to more complex (eg. science). If you're in a more demanding department and underperforming, you could get fired and have to apply to another job in another department.
For example, the military is sort of like a job guarantee in the sense that anyone meeting the physical bar is effectively hired. But any military member can get discharged.
Remember that 40% of americans can't afford a $400 emergency.
Also, it's not only, even not mainly, a social program, but an automatic stabilizer in the economy.
It gives power to the workers too, because there is always an, even if with low pay, alternative available.
I don't think there is a great track record of jobs being run by the government (or farmed out by the government to other organizations) as being idyllic workplaces.
Also a bs job is soul crushing and work-ethic destroying.
The only true power comes from being able to walk away from the control of anyone else, being able to simply say "no". For that, one needs a UBI.
I don't know much about the USSR, but I don't think they set everyone's salaries the same or distributed money equally, or any other nonsense like that.