Swashbuckle can fail to generate a swagger / Open API document with the message "The same schemaId is already used...". This post explains what that means, and offers a way to work around it.
For API endpoints that return multiple types, you can use inheritance with Swashbuckle to get create a Swagger / Open API definition featuring the variety of available types. Serving all these types is not the default behaviour. This post shows you how to opt in.
NSwag is a great tool for generating client libraries in C# and TypeScript from Open API / Swagger definitions. You can face issues where Open API property names collide due to the nature of the C# language, and when you want to use decimal for your floating point numeric type over double. This post demonstrates how to get over both issues.
Generating clients for APIs is a tremendous way to reduce the amount of work you have to do when you're building a project. Why handwrite that code when it can be auto-generated for you quickly and accurately by a tool like NSwag? To quote the docs: