How much UBI would be needed for most of the population to live comfortable middle class lives? UBI isn't going to be acceptable if it reduces the middle class to just above the poverty line. People want more than just having enough to eat.

So while it may solve the problem for people who are food insecure, it will create a problem for those who were better off with good paying jobs. You can't just reduce 95%+ of the population to simply meeting basic needs and not facing serious societal discontent.