Homebrew Cask's uninstall scripts are basically a community-maintained "best guess" at to how to full uninstall each piece of software. It's generally pretty reliable, and I do use it to remove non-brew installs sometimes.
Note: I have contributed casks to Homebrew Cask before.
You can add `--force` and zap will also work on non-brew installs. The paths are community-contributed, so watch out (you can print the paths with `brew cask cat <name>`).
Note: I have contributed casks to Homebrew Cask before.