iPad and Foreflight
Specifically the issue I've seen running ForeFlight on a wifi-only iPad is that traffic is depicted in the correct location on the map background, but ForeFlight (the iPad) doesn't know where your own aircraft is. It was an easy thing to check. My iPad has an internal GPS (wifi + cellular version). The iPads in question did not. I'd go flying with the customers in their airplanes with both iPads along. Mine knew where we were, the others didn't. Plugging a Bad Elf external GPS or similar into the wifi-only iPad would solve the problem.
Specifically the issue I've seen running ForeFlight on a wifi-only iPad is that traffic is depicted in the correct location on the map background, but ForeFlight (the iPad) doesn't know where your own aircraft is. It was an easy thing to check. My iPad has an internal GPS (wifi + cellular version). The iPads in question did not. I'd go flying with the customers in their airplanes with both iPads along. Mine knew where we were, the others didn't. Plugging a Bad Elf external GPS or similar into the wifi-only iPad would solve the problem.