First, my use case.
Symbolic links (still) aren’t available as a post-processing option, however filebot for example, handles them well.
I use both Usenet and Torrent and the objective would be to have normal importing for usenet files through Sonarr and custom post-processing through filebot for torrents.
This could be achieved by keeping Completed Download Handling enabled for nzb client and disabled for the torrent client which allows for custom post-processing script to run.
Options to signal the end of post-processing to Sonarr could also be implemented (maybe already are) to replicate the current flow.
In summary I believe Completed Download Handling should be settable per download client and not universally to allow for “On Download” script to be executed before Sonarr moves or changes the download.
Also disabling Completed Download Handling probably shouldn’t be a warning in the configuration but an information message.