I guess I'll put in a few more cents.
There is a lot of crappy RMGUIs out there, so I understand peoples frustration and need for something new.
But RMGUIs doesn't have to be that crappy, and IMGUIs, while solving some problems, overlook others.
There is no reason an RMGUI would have to have complicated observers or invalidation logic.
In the RMGUI I'm currently writing, if you change something, vertex buffers will be invalidated and rebuilt before the next draw.
If not, well, then we reuse the vertex buffers from last frame. (normal case for 99% of the frames)
Its not hard to know if a change will mean the vertex buffer it belongs to need update.
I don't care at all about regions. That was relevant in software rendering. Not so much in HW.
You do decouple creation from update/reaction, but you do it for a reason. It might not even be the same person defining it.
But even if it is, I like to have my widgets initial position and layout defined in an xml. Nice and neat and does not pollute my code with layout, and I can even edit it in runtime and reload it for quick ui tweaks. (or complete ui changes as long as the same actions exists)
Since I use lambdas, there is no problem with extreme decoupling, all my callbacks are defined right then and there, without unnecessary code.
Code should be concerned with actions, and never care about exact sizes or positions.
I don't want to rewrite logic for when and how to draw my buttons for every game I do.
It makes sense for game engines where you have complex needs for culling and handling of transparency and effects.
Not that much for UIs.
To have higher class objects like views and popups make code brief and easy to read and modify the relevant parts of.
In my RMGUI, my UI is just another layer/pass in our graphics engine, I call "update" on it when it should be updated, and I call "draw" on it when it should be drawn. (so in a way IM, but on a higher level)
But I wouldn't want to call the equivalent of "draw" for each and every ui item each frame.
Not because I think it would be slow, but because I have no need to see and change that code, it can be completely generic for every UI.
Then of course, RMGUIs can be made overly complex... like most out there.
But it doesn't have to be like that, and I'm not sure IMGUIs really solve the problem, just moves the responsibilty
I don't want to discourage anyone from using an IMGUI though, I'm sure it can be perfect for a lot of projects.