Why is it hard for you to grasp the fact that UBI will work under "Capitalism".
In my opinion UBI has a difficult time to work out, just for the usual problem, that where there flows money easily, money will be spent easily and that results in inflation. So the money that should help the struggling in the first place is increasing the prices for them as result.
Anyway we can not predict the economic implications of UBI with certainty, like QE should have resulted in more inflation, but actually it did only for certain assets, like real estate in major cities and stock prices. On the other hand the QE example shows clearly, that there are different outcomes with certain money flows. That is also on of the best arguments for decentralized crypto currencies by the way, that a stake holder/investor is safe of forced money flows that eventually will cripple his investment.