Duh, there is not a single automation that Smart Lighting can do that Rule Machine can’t do 
; of course the opposite is not in the least bit true. @alex said that lighting has to run locally, which I agree with. But Smart Lighting is not anywhere near being a robust lighting automation application. Alas, RM is stuck in the cloud.
I just finished replacing all but 4 of my SmartApps with Rule Machine. I still have things like Echo and Harmony integrations, and Button Controller+. All of the remaining automations are handled by RM, which wasn’t true until today, due to last night’s update. I finally realized that I could add the features that I need to RM, to get my needs met. Voila: mode based motion controlled lighting, with button and switch triggers, time and mode restrictions, and disablement options.
It certainly doesn’t matter to me what category they put Rule Machine in, and I think you are right that RM is not the easiest app to master. ST has long been stuck with this view that only simpleton things would fly. But, we don’t really know, do we? RM certainly has proven that many users want and need more sophisticated tools. ST needs the Marketplace for SmartApps to have the same ability as in the Marketplace for Things to link to manuals. Rule Machine needs its documentation. Right now, they don’t have a way to provide that link.
BTW, RM is not more “impactful” to the system than “smaller, specific apps”. It just happens to be vastly more flexible. Each Rule tends to have a couple of subscriptions, and a tiny set of actions, just like the small specific apps.