The whole idea that people are inherently lazy and won't work without being forced to always puzzled me. Most of the people I know want to do something productive, but more often than not it's either not something they can get enough income from quickly enough to be able to drop their day job and start doing it full-time or it's not something they can get enough income from to keep the bills paid. Give them a guaranteed basic income and they won't sit around doing nothing, they'll start doing what they want to do (instead of the day job they have to have because it pays the bills).
One catch is that there are a lot of jobs that noone really WANTS to do,but do anyway because it beats starving: untrained menial labor like cleaning toilets or picking crops in the hot sun, as well as backbreaking heavy labor like mining coal, etc. By effectively releasing a somewhat captive workforce from their NEED to continue doing those jobs, expect the salaries in such fields to have to rise dramatically overnight in order for them to remain sustainable when a large percentage of current workers say "screw this!" and quit. This would either lead to significiantly higher raises for some jobs, or could even make entire sectors and industries entirely non-viable when having to compete with other other countries without universal basic income.
Universal basic income will ripple through the entire economy: prices for a lot of products like produce grown in your own country are likely to increase significantly, while more spending money on the underside of society will also lead to an increased demand for certain goods raising their prices. If foreign-grown foods are a lot cheaper, you may end up killing your own agriculture industry and becoming almost fully dependent on other countries for feeding your nation: a dangerous situation to be in.
Whatever the determined amount of money would be, it may very well end up having a lot less purchasing power than people would anticipate ahead of time.
In the short term, i could be VERY disruptive to the economy, but of course only time will tell how this would play out over the long term.. Unfortunately it's the kind of thing that's hard to experiment with on any large scale, since having the revoke it if things don't work out could also have a potentially disastrous impact on many people's lives.