I have a similar experience as jakfish. While my x25 Gemini is a somewhat crude first generation product, the typing experience isn't the best (not sure if later batches are any different), and the lack of a proper rear camera meant I kept carrying my old phone around too, my Gemini worked well during the nearly two years I used it frequently.
Since then, it has, still in good order, been in storage, as I am now using a Cosmo, which came from the factory with tiny hairline cracks around a few of the magnets, and parts of the middle hinge has spontaneously fallen off, though neither has affected its usability. While I can't say for sure that the Cosmo is less durable than the Gemini, I can, at least, say that the Cosmo does not appear to be any more robust than the Gemini.
Both are clamshell devices and, as such, a bit more delicate than monoblock phones. As the small quantities means the Planet phones must be fairly cheaply made, by a third party, there are some quality issues with both, so whether you get a good one or not, seems to depend a bit more on luck than it would, were you to go with a major brand.
As the latest (last?) Gemini firmware available is from May 2019, I would, due to the lack of Android security updates, currently only recommend the Gemini for Linux use.