Author here. JSON decoding is hard relative to what it is like in JavaScript. In your JS code you can just call JSON.parse() and get the corresponding JavaScript object.
In Elm, decoding is not nearly as easy as it is in JS because every field must be explicitly converted to an Elm value. Depending on the complexity of your conversion from JSON to Elm value (e.g. whether you are just decoding to primitive values or to custom types defined in your program), there may also be a bit of a learning curve.
As I stated in the post, there is a benefit in doing all of this: your Elm application will effectively type-check your JSON and reject it if it is malformed.
No, that is not a joke. If one tried to convert an Applet to HTML5 and came across a server backend that returned serialized Java objects, parsing that into JS would have been significantly harder than the two lines with ObjectInputStream it would take in Java.
It's worth noting that JSON decoding is not _hard_ as such, but it's harder than other parts of the language. It is more time consuming. Tools like these can help reduce the amount of time you spend hand writing decoders.
In Elm, decoding is not nearly as easy as it is in JS because every field must be explicitly converted to an Elm value. Depending on the complexity of your conversion from JSON to Elm value (e.g. whether you are just decoding to primitive values or to custom types defined in your program), there may also be a bit of a learning curve.
As I stated in the post, there is a benefit in doing all of this: your Elm application will effectively type-check your JSON and reject it if it is malformed.