Immediate concern for me would be safety. My impression is the battery pack provides a lot of the rigidity necessary to keep the car in one piece, and swapping the battery out introduces new hazards.

There's also the concern about liability, whether the battery pack you're getting is as good as the one you're trading in (what if it's older and has a lower overall capacity), and how that might impact resale value.

Third would be logistics, keeping the flow of batteries through the stations and having enough batteries on hand to swap on demand, and keeping compatibility with different makes and models, is a challenge.