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>`).

popcon is opt-in.

No. Homebrew sends a per-installation unique identifier to a third party (Google), tracking your location across different IPs, whether you want it to or not.

Popcon is first-party, and is entirely opt-in. It doesn’t send anything unless you want it to.