The hardest thing about a flying car is not constructing it. It's already been made in different variations. It's about reducing its size for it to be able to ride normal roads (which I think is accomplished quite well in that model) and making a cheap fuel for it which would not burn completely within a couple of minutes.
The same can be said about personal jet devices. It exists, but it's far from being something you can carry in your pocket. It's huge, burns fuel like crazy and terribly difficult to control.