After using custom data types for several months I can officially say that the time for development and ease of linking front end is absolutely remarkable. However, since the only documentation in FlutterFlow on custom data types has a broken test api, it forced us to learn thru trial and error and searching the globe for answers ourselves.
Good news! Sometimes the greatest satisfaction is solving a problem or learning a tough new skill. Custom Data Types is an incredibly powerful tool that I would encourage more developers to utilize. This is particularly valuable when building with API's and more robust apps. The speed when compared to using json paths, or even predefined paths is tremendously improved. Moreover, as more complex json structures are utilized, the issues with correctly mapping data seem to be resolved significantly by using custom data structures.
Important note: When using Listviews and Custom Data Types there is an interesting behavior. If you change anything in the Data type or change which Custom Data Type to build from, you must delete the Listview and rebuild the Listview. We have run into this multiple times as the app improves, we could not figure out why the correct logic did not seem to work. By deleting the Listview and rebuilding the exact same logic works. Hope this helps some of you save tremendous amount of time searching!
Custom Data Types is a game changer. Take the time to learn it, test and play. If you have questions do not hesitate to reach out and ask for help!