Power Divide - society will be easily divided into two groups: those who depend on the UBI to live and those who don't. The former will be absolutely at the mercy of the latter.
One aspect of UBI is that poor people would be less at the mercy of the rich. If you're a poor person now you absolutely have to try to keep your horrible job else you'll lost the ability to pay rent and buy food. You have to take all the crap your employer gives you. With UBI, assuming it's enough to live on, you could walk away from that job and still survive. That shifts a lot of power from employers to employees. It's one of the most appealing aspects.
One aspect of UBI is that poor people would be less at the mercy of the rich. If you're a poor person now you absolutely have to try to keep your horrible job else you'll lost the ability to pay rent and buy food. You have to take all the crap your employer gives you. With UBI, assuming it's enough to live on, you could walk away from that job and still survive. That shifts a lot of power from employers to employees. It's one of the most appealing aspects.