OpenApiSpex is the continuation of the Phoenix Swagger project, so you use it if you want a generic REST API documentation.
I’ve used about half a year ago in a new project and it worked flawlessly, used it with built-in swagger-ui behind an authentication wall. The only main issue I had was that I didn’t understand how to use the specs at first, however it becomes clear when you read from official open api documentation the format of the specs.
