> wouldn't UBI, even if coupled to actually producing _something_ will lead to a lot of useless stuff being made?

The general premise of a UBI is that it's unconditional.

If you tried to say someone is required to produce something without specifying what it is, they'll produce whatever is the easiest thing to produce, which will naturally be useless if they otherwise wouldn't have produced anything because the only reason they're doing it is to satisfy the demand of someone not imposing any specific requirements on the output.

But if it's actually unconditional then the things produced would only be the things someone wants to produce, i.e. the things worth their time to produce when they're not actually required to spend their time producing it. Those things would tend to be useful because at least the author found them to be and there's a decent chance they're not unique in the world. If you e.g. make an app just because you want to use it yourself, maybe someone else wants to use it too.