I'm wondering what the difference between this and json-cpp https://github.com/open-source-parsers/jsoncpp is. They look like they provide the same functionality, albeit this looks more "modern C++ style"?
For starters nlohmann/json is fuzzed (look at the test directory). From my experience it is really stable. jsoncpp still has some quirks as it relies on unit tests only. Big no no for me.
nlohmann's is super nice. You can set a json object equal to a map or vector and it will automatically understand and do it. I doubt you can do that with jsoncpp.