I think those look really exhausting and unnatural since you have to carry your whole bodyweight on your arms.
Seriously, try suspending yourself with your hands on two chair backs. Now imagine doing that for many minutes at a time, while also withstanding heavy vibration and precisely controlling your flight.
It looks like the arms are just here for controlling the direction, the jets look supported by a belt attachment: https://youtu.be/aFQSFke0wFc?t=35
That video was one link down in the article, several other interesting concepts are shown, that generally seem to be platforms with thrusters.
I feel like a more practical design could be something a bit like "falcon" in the avengers: back-mounted thruster that gives a lot of forward thrust, with deployable delta wings à la jetman. Since the thrust is forward, you'd need something to right up for landing. Either with a physical movement, or aerobraking above the center of gravity (could be using the wing?).
For now, designs that only rely on thrust, and not lift, are probably doomed by their fuel efficiency and noise.
I remember watching an YouTube interview with the founder, and he explicitly said that he could suspend his entire body weight on his arms alone, on pilates or something, and that was the base idea for the design he went with.
He is a former marine and I think one would need considerable strength to use his jetpack.
I wonder what these guys could accomplish if they partnered with, say, DJI or something. Their flight controllers are very small and have years of development in automatic flight stabilization. Anyone that has flown a DJI drone knows what I'm talking about. It would take balancing out of the equation and make these much more feasible for a general audience.
If I'm not mistaken, the Gravity one has a jet mounted on the back too, and in fact it's by far the biggest one. The one on the arms is just for balance.
Some weight would be supported by aerodynamic effects once you start moving (I've seen videos where they have a squirrel suit-like web between their legs).
Holding yourself up like that is probably comparable in strength/endurance requirements to other sports like rock climbing, which still manage to be very popular. This design has a fitness prerequisite, which is okay. It's probably only the extreme sports crowd that would be interested anyway. If you want to fly, well... better start practicing those chair dips!
I think you're talking about alpine climbing or mountaineering. Rock climbing in the modern sense does not mean that, and overhangs (not undercling, that means something else) are very common.
Almost all gym lead routes are overhanging for fall safety reasons. Stemming/mantling (which would use the same muscles as the jetpack) is considered much less burly, anyway.
Seriously, try suspending yourself with your hands on two chair backs. Now imagine doing that for many minutes at a time, while also withstanding heavy vibration and precisely controlling your flight.