So a positive thing about UBI is it helps better determine the cost of cheap labor. For instance, you would have to pay most people way more than minimum wage to work a difficult job in fast food, but people work those jobs because of the low skill requirements.
Under UBI, those jobs would have to pay more in order to get workers, since no one will work a shitty job on min wage when they can live off UBI.
So the jobs in society that are valued the most are no longer the most skill intensive, but rather the jobs people don't want to do the most, which is both a positive and negative thing.
Some goods become cheaper, some goods become more expensive, potentially decreasing purchasing power.
Another side effect could be the acceleration of automation, which is a very good thing. If businesses can't get fast food workers in, they'll be forced to invest in automating those jobs or making them easier through technology.
In which case, products stay cheap across the board, and it doesn't matter if these people choose to stay unemployed because robots are taking care of the work for them, so society doesn't really feel effected. There's a principled argument against this, but from a utilitarian standpoint this is best for everyone. Unfortunately I don't think we're quite at that point. Maybe in a decade UBI will be able to have more universally positive effects like this but I don't think we're quite there yet.