They lack sideways control. However, for amphibious usage or sport usage, they have their uses. Wheels are still better. Jet cars are not good technology. Vertical Take Off and Landing aircraft are more efficient.
Helicopters are more maneuverable and are also efficient.
Tilting engine aircraft show much promise. Theoretically, they would beat out VTOL aircraft, but not helicopters. Note that Harrier jets and other vertical take-off aircraft do not always work as promised. They can only take off vertically if they are lightly loaded. If you are packing a good combat load, you will need a runway, albeit shorter than any other jet fighter.