We did a great many of them over the decades; I find the mongoldb ones we encountered the most horrible (even compared to vague 80s + 90s plain text formats as they usually at least had some documentation as to what the developer was storing and how), but in general, we found the most reliable and, when you are used to it, the fastest (in the end) is creating types for everything and convertors from one to the other type. We did try other (more sloppy) ways before it and they always kind of result in bad quality. We have noticed that making (very exact) types for everything when doing complex conversions works well. Most notably, we try to avoid stringy types and add a lot of validations in the deserialisers. You can run conversions 10 years later on new batches and still get understandable errors.