Contacttower
"I would have thought that since they are usually used in aircraft anyway there wouldn't be a problem."
To be blunt, but I hope not unkind, what you think is not relevant. Aviation legislation prohibits the use of any electronic/electrical equipment on aircraft for which that aircraft is not approved. The approval may be a general one, as in mobile phones, or may be more specific.
On a technical note, there are many aircraft systems that are susceptible to interference from electromagnetic interference "EMC", FADECs are a particular case. In the modern world, most electronic devices are digital such as laptops, GPS, mobiles, calculators, etc., and clearly an aircraft and its systems cannot be tested for interference from all specific devices.
You can imagine the case where "Doors to Manuel" is sat in the back of a nice modern twin donk and decides to switch on his GPS. Immediate effect, both FADECs cough and splutter and the wicks go out. As far as I can see, the only good thing that will come out of the scenario is that he will know precisely where the crash is.
Bottom line, any equipment to be used on an aircraft must have been tested and proven not to interfere with aircraft and its systems. That is an expensive business but cheaper than finding out the hard way.