UBI makes sense for a purpose of bringing more buyer's/actors to the game. It's especially useful considering our dependence on jobs as the primary activating mechanism shows strain under the troubles of scaling human coordination and hiring. In a sort of backwards way we get more jobs when more people can contribute to the flow of money.
However UBI doesn't solve the problem of debt still piping the cash back into the hands of banks and other financial institutions. What are these UBI checks going to be spent on? Rent that's too high? Student loans for indulgent tuition prices? Without blocking the pipes from the poor to the the actors with pipes of their own, these throughput problems aren't solved more than delayed.
An actor is still a single actor, so if one actor has an over aggregation of wealth and has trouble spending it effectively the potential of that wealth is wasted when at least some actors don't have enough.
So surely trillions of dollars of UBI injected directly into the hands of the poor will stimulate the economy. But for how long until the cash starts to aggregate again into slower pools of cash where throughput is limited? In a sense we almost want inflation if these "overaggregators" are so abundant, but only if the population is maintained at a base level of wealth relative to the inflation. Otherwise the rich, who probably got rich by being more effective with their money respond to the market changes faster and the UBI stimulation is only temporary.
But if UBI causes significant inflation, and the pipes going directly to every individual have enough back pressure, it could be a great situation where the overaggregators lose value to inflation as they struggle to spend or invest.