Business rules are very powerful when you want to have a re-useable piece of logic, for example to validate a string field. In Microflows they can be used, but not in nano flows. This is strange, because nano flows are perfect to use for validation logic.
You got my vote.
When using regex, the function calls the regular expression language provided by the current platform, which is different for microflows and nanoflows. Therefore it would be neat to have a boolean available whether the rule has been called from a client or from the server.
Additionally, if we are able to use calls in the client it would make sense if we could use Rules for expressions for visibility, editability, text templates, and dynamic classes. Imagine the power if rules could be made available as a string function