during taxi mode no problem lah.. but not during take off and landing.

it all depends on the electronic design of the device. some emits signal with high frequencies which may cause intereference.

maybe the signal come from that part that's handling user input (touch pad, lack of debouncing signal), output (audio), or even power supply. The thing is, it's hard to design a fullproof electronics circuits. It can and should be done, though.

anyway, it's a good thing to advice everyone to switch off their electronic gizmos since maybe quantitatively it may harm the flight.