I think providing location/COL adjustments would be a big mistake for a UBI system. Providing a flat rate encourages people to move to lower-COL places which would reduce the amount of economic activity tied up in rent. It seems to me that adjusting for COL would just reward those who choose to prop up rents and worsen the rent <-> COL cycle.

Agreed.. the main benefit to such a system would be simple rules that apply to everyone, that you can't cheat (other than potentially non-citizens getting benefits). It there's any complexity, including COL, that makes it easier to cheat said system.

Indeed, by encouraging people to move to lower cost of living areas, they may actually help prop up the local economy by spending their UBI cheques in otherwise depressed areas.