I was just playing around with the email integration, which I was hoping could be used for processing emails we get in a shared inbox some of our content team have to manage. However, since there doesn’t appear to be labels sync’d and you can’t create rules that operate on the body of the email, the use case is limited.
Triggering off of rich text field changes would require more work-arounds, but there is at least a work-around for an initial processing of the rich text. You could leverage a script to execute on entity creation or update that could search the contents for keywords or whatever, then apply relations based on the contents, copy text into other fields, etc. However, this would be a lot easier if you could just create rules directly on raw, HTML, or markdown as you suggest.
It also would be nice, even if you couldn’t directly apply rules to the rich text, but to at least have a trigger of when it is updated. You could then trigger a script to run and apply whatever changes to the entity.