I guess for "Flugzeug", 'device' works better but I don't think I have ever thought about it in that way. From my experience usage usually goes more like this: "Pack dein Zeug zusammen wir gehen weiter." which I think best translates to 'Stuff' or 'things'. Zeug is an interesting word though. It is also used for the harness of a draft animal ("sich ins Zeug legen"), it's bridle ("Zaumzeug"), bed linen ("Bettzeug"), work tools ("Werkzeug") , or as a word for nonsense ("red kein dummes Zeug"). I would say it started, as you said, as a word for device but became a slightly negative word for 'Stuff'.