One thing I do not see with Stainless is also using it to generate the server side of the API.
We use OpenAPI as a design doc, and generate client SDKs and Datos for the server from the same spec. This gives us pretty solid control over interoperability.
I agree, I love them in theory, but wrangling mustache templates is a hassle. They end up super coupled to the code that populates them which makes them a bear to modify.