"More difficult than you might think" is a far cry from "not possible." The press has been gleefully reporting "experts" and official spokespersons reassuring us that the IFE and critical flight control systems are "separate" systems that are not interconnected. However, in testimony before the US Senate, United president and CEO Jeff Smisek indirectly acknowledged that critical flight systems and passenger Wi-Fi are, in fact, interconnected. "There are clear firewalls between a Wi-Fi system and any kind of control." Firewalls, of course, can be breached, which is what hackers are good at.
Boeing actually deliberately designed passenger cabin and flight systems on the 787 DreamLiner to share network components because it enabled them to save some 900+ kg, meaning there are any number of possible crossover points.
Digital hijacking of a commercial airliner is an unlikely scenario but denial will not make very real and demonstrated vulnerabilities go away. Before Roberts flamboyantly grabbed the spotlight, other security researchers (Teso, Santamarta among them) had demoed techniques to accomplish what he has claimed.
--Prof. Larry Constantine (pen name, Lior Samson, author of Flight track)