I am probably misunderstanding your point BUT I have actually depended on Nix for "reproducible docker images" for confidential compute usecase so that all parties can independently verify the workload image hash. Rarely (actually only once) it did fail to produce bit identical images every other time it successfully produced bit identical images on very different machine setups. Granted this is not ISO but docker images, but I would say Nix does produce reproducible builds for many real world complex uses.
Ref:
[1] https://gitlab.com/prateem/turning-polyglot-solutions-into-t...
[2] https://discourse.nixos.org/t/docker-image-produced-by-docke...