You just know this is going to be 2-3 times the price of an economy seat, which means you're going to buy the economy seat anyway. In flights, price beats everything else.
I think this is an ancillary specifically for economy class passengers—you pay a certain amount to reserve a 4-hour slot on a bunk or something. I have no idea, but my guess is economy + nap slot would still come in well under the cost of a business seat. Plus they can sell those same 6 berths what, 4 or 5 times over in the same 22-hour flight?
If I recall correctly they’re not certified for use during taxi takeoff and landing, I’d imagine due to orderly-evacuation kinds of requirements.
You could pack a lot more in if you sedate them and put them in racks like they have at the morgue. TBH I'd sign up for that on a long flight if the sedative recovery wasn't bad.
https://www.airnewzealand.com/economy-skynest