The great thing about Codable for simple types is that no code is required. But the slightest need for something not simple requires a lot of boilerplate code. Being able to accomplish less simple things with no code should be a goal. Here are a few pain points for me:
Dates! The default of Date decoding to seconds isn't sensible for JSON. The lack of support out of the box for fractional seconds has been a problem for me. It is possible to have an internet date parser that accepts all forms of international dates. I use SwiftDate for this but it requires custom code.
Default values. I guess there are property wrappers for this but I haven't investigated them. It should be possible to supply default values directly to the decode() methods. It should also be possible to provide the default values in a more declarative way, so no init method.
Future proofing enums. I commonly have simple enums, either String or Int based, where the server API adds new cases from time to time. Any given version of my app only knows about the current cases. The only way I deal with this is try/catch with decoding in the try block and assigning a default value in the catch block. If I forget to add this then parsing fails if there's a new case.