I don’t really care how I populate the Things and Items. While a line of configuration is somewhat easier to spew out than JSON, neither is all that difficult. Single-line configuration is easier to see diffs in git or the like, but if it is all programmatically generated, then I can always look at the single-line source files that drive the scripts.
As for defining rules, an IDE at least gives completion of Item names, which can be very helpful in not only speed of authoring, but accuracy as well. Since I want to be able to read my rules and only the Thing ID appears in the rules, a lot of information “needs” to appear there, so they become things like “pico_Bedroom_Entry_on” or “group_front_of_house_only” – many chances for missing or mistyping characters.
An IDE, be it in a stand-alone application or “web-based”, also should provide only the “acceptable” alternatives for things such as object methods and signatures, as well as indicate type of return values and call parameters. I’m used to includes/imports being generated automatically with eclipse, IntelliJ, and others. This especially when the language in play isn’t a well-known language. I’m not trusting Designer too much at this point, but that is the expectation when a project says it is based on eclipse and eclipse frameworks.
Trying to get everything configured using file-based approaches was also based on your own approach and suggestions in Eclipse Designer -- Unable to "see" any PaperUI-created things/items