For some reason right now extensions decide where to use and where don't NFEL - that's bummer. It makes complete automation impossible and DevOps unhappy.
I think it will be good to twist this not a good practice and give users more freedom in using Expression Language / Variables whenever they want! And I think it could be solved by introducing forceEl flag for PropertyDescriptor, that could be set by user and saved with property value.
With the change we can deprecate the old complex and non-intuitive setting support of EL in the extensions and even set forceEl by default for some extensions that already supporting EL scopes.
I'm already working on the first implementation here: https://github.com/apache/nifi/pull/3332