It's fantastically expensive.

So, ~4.4% GDP is pure healthcare waste. Social Security is 4.9% GDP and welfare is another ~4%. So, right now we could use 13.3% GDP or 7k per person in the US. However, that 7k would remove the justification for a lot of low income tax breaks freeing up enough to hit the 1,000$ per month range without raising net taxes.

Now, this would take a huge shift at the federal and state levels, but compared to say the 'New Deal' it's not actually that large a change in policy. Further, while some people would stay at home, it would also free up a lot of jobs and inefficiency.

The real problem is a small but very influential slice of the population would be worse off.

PS: Remember, every policy change is going to make somebody worse off, so harming someone is not enough to make a change bad.

Sounds like a fantastic job for machines! Humor aside, even with UBI you will always increase your income and wealth if you put in extra work, unless some current schemes where your income stagnates or even falls if you earn enough to put yourself outside some arbitrary bracket.