The Z-Wave alliance guarantees interoperability though their certification process on their main site:
Z-Wave encompasses a broad ecosystem of smart products and services that work seamlessly between brands and versions. This interoperability, which has been the hallmark of Z-Wave technology since 2005, is achieved and maintained through Z-Wave certification, a testing program administered by the Z-Wave Alliance consortium.
Z-Wave certification ensures that all Z-Wave products work together with each other regardless of brand, including backward-compatibility between versions. The certification process includes technical testing, programs for uniformity of marks, and enforcement of the certification standards.
While other technologies claim interoperability, only Z-Wave offers interoperability at the product level. This ensures manufacturers, integrators and end users that their products and services will work together with all certified Z-Wave products.
They mention testing, but really don't go into to much detail on what it entails to guarantee interoperability. What I'm wondering is how the tests the Z-Wave Alliance uses are designed to guaranteed interoperability and if the claim to interoperability is actually true.