1) Realistically, you're going to have to raise taxes to fund it, not just deepen deficits
2) You're going to have to cut many social programs as well that overlap with basic income. For some people this isn't really a good trade-off. I saw one blog-post proposing a long list of social programs such as disability support and wheelchair funds, and a local activist parent of a severely disabled child pointed out that this would devastate his family financially.
I get the appeal of an anti-bureaucratic one-size fits all solution to the social safety net, but I'm skeptical.