POST JSON data denormalization error
What has always worked for me when I build custom REST resource plugins for 8 is making the request and payload for the request in HAL+JSON format after activating the format for the endpoint. The return data has always come back as just JSON. Since many times I've already built content entities which have their own GET, POST, PATCH, DELETE operations, I generally build the custom REST resource plugins for custom operations which I create. I've only tested overriding the default core operations a few times and then I had to disable the core's REST endpoint on a specific operation to allow my endpoint to take priority. ie. My POST operation is used rather than the core's. I'm currently using the Beta14 version of 8.