I have used the Nexus 9 tablet, iPad
Air 2, iPad Mini 4, and Nexus 5x (phone). They all work for the most part but some quick thoughts:
Nexus 9: screen is too big (hard to fit into the hardshell backpack), app is solid, everything works well. ($399 msrp, $325 street).
iPad
Air 2: screen is too big (hard to fit into the hardshell backpack), app is solid, everything works well. ($499 msrp, $399 street)
iPad Mini 4: screen is perfect (fits perfectly in the hardshell backpack), app is solid, everything works well. ($399 msrp, $350 street)
Nexus 5x: screen is too small, app has issues (sometimes I get no video signal - usually unplugging and plugging back in the cable or killing and restarting the app fixes it). ($299 msrp, $299 street).
I actually like the Nexus 9 the best but I'm more of an android guy I guess. The ecosystem has lot of free apps to record the screen, connect your tablet to your computer and browse through the file system to download cached videos, etc. The app is solid without any crashes, etc.
iPad
Air 2 is fine but is a little too big and heavy. It feels unstable and seems like the tablet is going to fall off. You need to open up the tablet holder to the maximum size to get it to fix. Similar to the Nexus 9, it doesn't fit in the slot in the hardshell backpack for your tablet so you need to store it behind the top foam layer.
iPad Mini 4 is the perfect size because you can easily read everything and it fits perfectly into the little slot in the hardshell backpack.
Nexus 5x is a nice phone but not a great device to use with the Phantom 3. It's too small and the app is too buggy. Many times I get a good connection (I can fly the plane) but no video. It also uses a usb c connection on the phone so you need to buy a usb cable (the one that this phone comes with won't work).