The comment you're replying to describes concerns about maintenance burden from the programmatic nature of Nix.

I think "declared in YAML, pointing to a Docker image", and "declared in Nix, pointing to some more Nix elsewhere in the codebase" is relatively moot to this point. (If you wrote the image, you'll know what it's expected to do; if you don't know the Nix you're pointing to, it's as opaque as a Docker image).

Configuring with a program vs using a plain structured file have different trade-offs.