The biggest flaw in any kind of UBI is the human condition, plain and simple.
Just as communism was too utopian a vision, so too is this. It can't stand up to the simple truth that people WILL cheat, and when one cheats, so will more until it collapses under the weight. Plus there's no real way to stop the cheaters because any measure you take to stop cheating can itself be cheated (because in the end there WILL be a human element to it; even robots have a human behind it at some point).
Sadly, the human condition means we blind ourselves to the reality we're entering Cold Equation territory: as in, barring a deus ex machina, there's no happy solution to the problem (for those who disagree, get twelve people to survive a week in the desert on just two liters of water). One way or the other, the likely outcome is some sort of population crash: be it by natural or manmade means. The blindness comes from the fear we'll be one of the casualties; it goes against our survival instinct.