json2nd uses a fairly lazy JSON scanner rather than a proper parser. Thus it may not behave fantastically with invalid JSON. If you’re having odd issues it’s worth running your file vs a rigorous JSON parser, such as jq’s.
Additionally not much work has been done so far on the wider reaches of the utf-8 character set, it should be fine with it but may report some errors strangely as this is a very byte oriented tool. Patches (that don’t make performance sacrifices) welcome.
There’s probably also corners of the JSON spec we haven’t got to yet.