Ok I'll bite, what kind of stoneware did they decide to make these out of that they have a 350F max temp?? Most ceramics can't even be properly fired at temperatures that low. It's a scientific marvel, my suspicion is these are made out of hardened play dough harvested from local kindergartens, and given a pretty white coat. I'm suspecting that it's a caution due to the metal handle on the lid expanding more then the ceramic and potentially cracking it but without guidance it's just speculation.
But with a 350 temp range what can you actually do? The creme brulee they list? Good luck broiling or torching the top without this apparently turning into shrapnel on you. Mac n Cheese? What kind of blasphemous creature doesn't crank the heat to get a good browning on the top layer? The answer is no creature, even Lucifer insists his mac n cheese finishes under the broiler. If the devil wanted soft and smooth with no crispitycrunchities he wouldn't bother putting it in the oven.
The most useful thing I can think of for these is making tiny cheesecakes. Low temperature, cute presentation.
Lodge has a set of these, about half the volume; 10 oz vs 16. Which in my opinion makes them much more feasibile for personal desserts and adorable presentations, but they're enameled cast iron and are safe to 500F. Still not broiler/torch safe, and I would prefer raw cast iron. Plus it would probably be cheaper for them to do as well so I don't understand why they don't supply it. Currently going for 43 on bezosland, so a bit pricier but at least lodge is a well known company.