I’ve worked on unionizing businesses with multiple locations and the big issue is communication. Locations rarely have overlapping staff below the management level and chatting with coworkers, having rapport with them is key to building faith in any sort of collective action.

That said, Starbucks Workers United (mentioned in the article briefly) may be able to pull something off with a more regional scope, but AFAIK they focus on shops that are already working on unionizing rather than cold calling locations.