This doesn't really fit in with the survey but I found that manually editing JSON files for complex themes soon become unwieldy. Even with my closures indicated by my editor I soon get lost in the nested elements.

Rather than a manual JSON edit it's easier of course to create the layout in the (new) WP theme editor and export the JSON from there, that process is actually very simple. Thankfully.

But it demonstrates my point:
I think theming in JSON was chosen for it's simplicity and legibility - which seems true at first but once we start building complex themes it becomes labyrinthine and really relies on our tooling to create coherent valid outcomes. So with that reliance on tooling the output format becomes a black box, and could have been any file really! It could have been SQL!

I'm not sure I have an actual point, just that observation: JSON loses legibility after a certain depth of complexity and loses the supposed benefits of direct text editing, it starts to rely on the WYSIWYG JSON theme editor. Which may be fine. It depends on use cases.