One of my customers flogs some device that takes card payments. He must be doing well because he lives in a huge place, indoor swimming pool, stables, underfloor granite-floored heating; granite work tops in his huge kitchen, etc.
Anyway, he tells me the mobile devices aren't very good because they'll only work if you can get a mobile signal. The ones he flogs are small hand-held devices that work without a mobile signal.
That's all I know; I personally don't see a need for it.