There are few advantages that come up as your code logic becames more complicated:

1. You might want to send the same object to different endpoint which might not support json but xml content type. So you can simply have one pojo and RestAssured would take care of all conversions.

2. Your object might change in runtime. So you will have to introduce changes to your string accordingly. This is quite an error-prone way. Serializer would assure that you send somewhat what is a proper json considering syntax stuff, escaping what needs to be escaped and so on.

3. There might be the case when you fetch object from one endpoint and send it to another. Hence you would be able to use class for deserialization and further serialization in runtime.

You need another POJO that represents a "CustomFields". For example:

public class CustomFields {
     private String Id;
     private String Name;
     private String Value;

     <getters and setters>
}

and use List<CustomFields> CustomFields instead of String[] customFields. Also note that your JSON example doesn't use camel case for the property names so your POJO shouldn't use camel case either.

There are ways to avoid using a POJO at all in REST Assured. You can for example use a HashMap instead which could be easier in some situations.

Try the approach like this:

MyPOJO myPojo = RestAssured.given()
                .get(new URL("https://YOU_URL"))
                .getBody()
                .as(MyPOJO.class);

And then compare the object to your golden one as usual.