It's the "U" part. Taking money from the middle and upper classes just to give it back to them is inherently inefficient and entirely pointless.

Additionally, I disagree that it has been researched to death. People have infinite wants, and we have finite means. I would personally rather see this play out at a smaller level in reality (not a time limited trial that biases the outcome) before we go all in on 300 million people. And if we are not going to be replacing existing programs with it, I would rather not see it happen at all. That's my personal preferences based on what I currently know, at least.