I'm getting familiar with Rule Engine after a few hours.
But I think a few simple UI changes would help new users "get it" more, and make it easier to work with visually.
Somehow highlight the "thing" you're currently working on (or "dim" the stuff you're not)
If you're editing a Rule Expression it shows everything on one white page with a duplicate of the expression you're working on. But it's difficult to "see" that you're in the context of the expression.
Keep script-blocks visually identifiable. So proper indents on multiple peer-conditions/actions. And then show the current evaluations off to the side, aligned together. So example:
Using CSS and JavaScript could be one way of overcome the limitations of the native UI, handling the all editing of the RM "code" locally in the browser.
However, to do that, Hubitat would have to made available APIs to handle RM rules from JS browser code. It would open limitless possibilities of creating/editing RM rules. It could be implemented, for example, receiving an JSON string with the rule and, after edited, send it back, waiting for the validation from RM engine of the edited JSON. Not to mention some others API calls requesting available devices and so on.
But - there's always a but, no? - the way we interact with RM to build rules I don't think that could be easily implemented on the HE side.
Let's see what Hubitat has on their sleeve for the future ...