I tried out the first version of the eyefi card many years ago. It was fun the few times I, slowly, got a picture from the camera to the iPad. Did not use it much.
Now I recently started looking at the F-stoppers DVD about interior/architecture photography and they mention they use real tethering (with camera control) on the iPad via CamRanger usb-dongle connected to the camera. Reading up on that I see that the hardware is from 2013.
So my question is now: what is the best way to tether a D800 to an iPad today?