Hi Roberto,
Serene foam is one of the "newer’ generation of high-performance polyfoams, which are a higher density and good at pressure point relief, but does not have any of the viscous properties (it is not a memory foam). It is also not a latex foam. Carpenter introduced Serene in 2015 and re-launched it in 2016. It is generally a higher density (2.5 lbs as an example) and it is CertiPUR-US certified (but polyurethane would not be considered a “natural” product). This generally would be a good quality and durable plush material.
Regarding durability, Costco doesn’t list the density of the polyfoam core in their 10" Comfort Tech mattress (or the older 13" that they used to offer), so there would be no way to determine the durability of this product. And regarding the “reviews” mentioned by Sweet Dreams on the Costco site, those would be very unreliable indicators for the appropriateness of the mattress for any one individual, nor a reliable indicator of quality or durability, as there are far too many variables involved that go into determining performance/durability besides the density of the foams, including firmness, position within the mattress, layers combined within the mattress, thickness, formulation, usage, mass of the person using the product, foundation used under the product, etc. – see post #2 here). Your best course of action would be to learn of the density of all of the componentry within the mattress in order to begin to make an informed decision.
As for “safety”, the only reliable way to assess the “safety” of different materials in more general terms is based on lab tests and the certifications they have for harmful substances and VOCs (regardless of whether they are organic or natural or synthetic) so that you have some assurance than the VOCs are below the testing limits for the certification. If the materials in a mattress or the mattress itself has a reliable “safety” certification then for most people they would certainly be “safe enough”. There is more about the different types of organic and safety certifications such as Oeko-tex, Eco-Institut, Greenguard Gold, C2C, and CertiPUR-US in post #2 here and more about some of the differences between organic and safety certifications in post #2 here and there are also some comments in post #42 here that can help you decide which certification is the most important to you for environmental, social, or personal reasons.
Phoenix