I know very little about custom rules, but last afternoon I did a few experiments with them.
My overall goal for today was to experiment to change the clearance constraint for tracks. Close to pads the clearance must be small so the track fits in between other pads, while for longer connections, I want a wider clearance to reduce inductive coupling between tracks.
At the moment it looks like such custom rules work on themselves, but the interaction with applying them dynamically during routing does not work very well. These custom rules tend to block most attempts of the interactive router to shove tracks aside.
I wrote a part of the things I found down in this other thread: How segment & footprint move? - #31 by paulvdh
In my second attempt, I did not add extra rule areas, but I turned it around. Default settings for the netclass has a wide clearance, while I used a smaller clearance inside the courtyard. The results were a bit easier to manage, but again, the interactive router was not able to shove a bunch of tracks aside anymore.
But also, I am now working with KiCad V 8.0.4 and I have not done a serious project with it. The interactive router behaves a bit differently compared with previous versions, and I am not even sure yet whether the results I see are because of the KiCad version, or because of these custom rules.