...and, the moment you declare you handle json, people will send non-string data ("that is a number, of course it isn't quoted"), attempt to include nested data, leave out the opening and closing [] (because people will grep a file with one array per line to filter a json file; that is no way robust, but people will do it, anyways)

The main advantage of json vs csv is that there is only one json, but quite a few different variant of csv. That is a huge advantage, but csv is engrained; there are many, many tools that handle csv but not json.

And the "if you control input and output" case is not the interesting or the problematic one.