Thinking about this a little more, I was worried you might have been right about nested JSON objects, and that deeply-nested JSON objects in the JSON-RPC API could still cause the thread to die because of stack overflow.
The parser does have a check that you can't nest JSON objects or arrays more than 100 deep: https://github.com/steemit/steem/blob/9e83f66c85a2c76bef1a07cef7dd302d2c4be572/libraries/fc/src/io/json.cpp#L442
But I'm not sure it's effective, I can think of one way it might be fooled.
RE: Steemd 0.20.6 bug - memory exhaustion when parsing malicious hello_message