Just an update: I have used the pubkegs for several fermentations and am hooked on them. I have not used them for long-term storage, but have no worries about several months storage in them. They have a PET plastic bonded with an O2 barrier plastic.
I have NOT used them pressurized.
I have used them only with a rubber stopper + airlock. No couplers involved, it is as simple as a glass carboy but with no stress about breakage and no stress about the cost of replacing it.
In response about plastic pails, most food sources in Japan seem to use tins for delivery. Familiar white 5gallon pails that cooking oil would arrive in for NA restaurants is replaced by square tins. Buying a bucket costs roughly 30 bucks and these kegs are free from any craft beer place. The craft beer places have to pay to get rid of them so are more than happy to give them away for free.
On another note: I noticed when serving from them at the restaurant I work at, there is often plenty of yeast in certain breweries' pubkegs... If I were interested in doing it, I suppose I could serve off of one, then carefully depressurize and remove the valve and harvest the yeast. Some people harvest and build starters from bottles, but this would be a hell of a lot more yeast. I should add that liquid yeast in Japan is also expensive and has to ship from the US or Europe, and paying to dispose of the plastic pubkeg is cheaper than the cost to buy a vial from whitelabs.