[ 1/5/20 ]
Agree with Jeff.
They are inherently dangerous, and vulnerable to single point of failure with fatal consequences.
With fixed wing or rotor wing aircraft, if the engine fails you can still land in a condition that would allow you to walk away, if you execute the required skill set appropriately.
In a jetpack, there is a space where you are too low for a ballistic parachute to arrest you, and even an airbag style envelope can cause problems if it deploys too low, and ends up projecting you on a high ballistic arc, with a very unhappy landing at the end of it.
Just the risk profile, and the need for ultra-high reliability on the jet units, means they are unlikely to be a commercial success, ever (except for use over water, or where higher risk from other sources overwhelms that risk).
The only way out of that would be to have multiple reliable engines, with the ability to land on any subset of them, and that will be expensive.