Wrt reproducibility: NixOS minimal ISO is now 99+%, GNOME ISO at ~95% (measured heuristically as described there): https://reproducible.nixos.org
Didn't realize they have gotten this far already!
The actual problem lies in the code that is packaged... A dev sneaking in malware in the CI/CD pipeline would paint a huge target on their back because it's easy to detect and normally few well-known members get to control this. Including malicious code in an obscure dependency however...