Great idea but..
That's usually people's response. I disagree. The only time it's a great idea is if your problem is "The world is way too rich. How can we possibly share out this bounty?" and that's not our problem. If all we had to do was feed ourselves we'd be in that position, producing as much food as anyone could want cheaply is pretty much solved. But we also have to provide healthcare. Trying to prevent mortals from dying can always absorb as much money as you can throw at it without ever being solved.
If we're not too rich then the fundamental problem with UBI is that it spends a mindblowing amount of money while actually making the most needy worse off. If you try to solve the 'making the needy worse off' problem by running a parallel means-tested system then you still have most of the problems of UBI but now you get none of the simplification benefits.
It's a very appealing idea, I get that. But it just doesn't work, even in theory and that's before you start to get in to fraud, etc.