A "low-income slum" is not a problem so long as people can live decently. It's "universal" so everyone receives it, what differentiates people in low-income areas will be that they inherited no capital and have no means of acquiring capital which is a problem that already do exist, so I don't really see your point.
Areas that are expensive to live in also have a need for jobs like cleaning the streets. If people have the choice of living somewhere cheap on UBI then the pay for lower skilled jobs in these expensive areas will have to rise to get people to take them. I would expect the cost of this to have to come from higher property taxes in these more expensive areas which could act to bring down the difference in costs between the different areas.
A person receiving UBI who lived in San Francisco proper and got free housing would essentially get 5x - 10x effective benefit than someone living where rent is $500 / month. Cue the inevitable conflicts over who gets to live where, and caps on what level of UBI will be provided.
But conversely, if the UBI benefit were a fixed monetary amount, then you'd be consigning UBI beneficiaries to never live in an expensive area where urban professionals work, so you would create a geographical and social divide where UBI recipients are consigned to live in low-income slums.
I don't see any way to resolve these dilemmas that would make all people happy, unfortunately.