No safer than any other method of running arbitrary code under your user account, or than any other package management tool with reasonable conventions about where it installs software.
If your goal is something that can easily and obviously be uninstalled completely, the App Store version sounds like exactly what you're looking for; Apple is pretty strict about what an App Store app can and can't do outside its sandbox.
Otherwise, I've been using the binary installers from PostgreSQL.org on OS X for years, and they work fine. They install everything into /Library/PostgreSQL, so they're self-contained, and the server starts/stops/can be set to auto-start easily with launchctl.
If your goal is something that can easily and obviously be uninstalled completely, the App Store version sounds like exactly what you're looking for; Apple is pretty strict about what an App Store app can and can't do outside its sandbox.
Otherwise, I've been using the binary installers from PostgreSQL.org on OS X for years, and they work fine. They install everything into /Library/PostgreSQL, so they're self-contained, and the server starts/stops/can be set to auto-start easily with launchctl.