I think 'arbitrary' was actually the key word. I agree that serializing/deserializing structs is pretty painless, the issue is when you're not sure what you're getting.

In golang recently I had to take some json (that I only knew part of the structure of), and modify just that small subpart of it without touching the rest. It was a really painful thing to develop, and the code ended up very messy.

There were a few golang libs for reading arbitrary json, but none supported writing to it that I could find.