Though I'm also not an economist and lots of economists seem to like deficit spending. So I'm just going to say I'm naive here.
I get that one of the benefits of UBI is that it's supposed to empower people to use the money in the way that satisfies their needs best, rather than rely on inefficient bureaucracies to determine what needs are worth subsidizing and who qualifies.
The problem is, there's no free lunch. It seems to me like any sensibly funded UBI is going to probably negatively impact many middle-class folks. Politically that's just a non-starter in the U.S.
I still do not buy the argument that a UBI will be shouldered by the middle class. I have yet to see evidence that it will be shouldered by anyone but the 1% (really the 0.01%).
Let's not try and justify bad ideas by pointing out that people currently do bad things.
It's all very well and good to say "look, virtually everyone would see a net income increase!" until you realize that's only possible by literally just printing money.
The costs of such high deficit spending are pretty certain, but the benefits meant to offset those costs are only speculative (and highly speculative, at that).