it costs a metric crapton of money, which does in fact need to come from somewhere*
inflation is a thing
UBI being universal means that some fraction of the money is going to stupidly rich people who don’t actually need it
could increase exploitation of elderly/vulnerable people
most of the other arguments against it are worse imo, e.g. some people believe that people working is like inherently good and that them being able to choose not to work is therefore bad
* some people say “just tax billionaires and fund it that way.” I am unclear if this is a serious policy proposal but assuming it is, even if you could tax 100% of the wealth of all American billionaires no matter how illiquid it is, that gets you about $4 trillion dollars, which works out to around $12,000 a person. Which could maybe work for, like, a year? But then you need to figure out how to pay for it the next year, and … you already took all the billionaires’ money.
(to be clear I am in fact pro-UBI, I just think it’s important to acknowledge the actual challenges with implementation)