The command line settings are a really useful tool, because it allows you to edit the video without having to use another video editor, because some things are too tedious to do in a video editor. However, for some reason you can at most input 500 characters into that box, which severely limits how useful it can be.
The reason I need more characters is that I made a program that generates a health log of players in the game, and I'd like to display this info on the Replay. Syncing everything in a video editor would be far too tedious so generating some FFMPEG input options is far simpler. However, these things get complex quickly and I almost always exceed the character limit. And honestly I don't see a reason to add such an arbitrary character limit anyways.