I was at a place 7-8 years ago with a hand-rolled JSON parser that had been written across a few afternoons by one guy without much ceremony. It was untouched after that aside from some renaming and other minor refactors. As usual with these things, the decision to hand-roll was made by one very opinionated dev who got his way because the business depended on him.

When that article was published it started an argument among the team. The guy ended up implementing the test suite to defend his decision. To my surprise the parser passed all tests apart from 3 or 4 unicodey-ones where it was more permissive than it should have been. There were plenty of production bugs at this place but none of them were due to the parser.

IMO parsing JSON really is simple. If you read through the article you'll see that most of the test cases are dead simple and hardly landmines.

This is not claimed by the article afaict. It's stated though that json spec/syntax is simple, and that it's simple as a language; my interpretation is that this means "simple/understandable/predictable for humans".

JSON is a subset of YAML. (you can just dump JSON in your YAML document and it’s a valid YAML.)

So parsing YAML will be strictly harder, as it needs to include a JSON parser.