If you're using Windows and there's critical stuff on your system, you should be worried about security because of Windows itself, regardless of fitgirl repacks.

I can't answer for sure, but admin privileges could have something to do with:

• Maybe overriding DRM functionality
• Maybe overriding limitations on what system resources can be accessed/controlled without the right developer's signature (so the game can go fullscreen, change resolution settings, etc)
• Maybe stealing your data or reporting you for piracy, especially if the authorities or AI malware or something had recently compromised the "fitgirl repacks" identity before the repack was released - can reduce risk by setting a minimum time e.g. 2 years before you'll use a torrent, or only installing on air-gapped systems or something