I'm having a hard time parsing -- you migrated everything from nix to containers (containers removed the need for nix) or to nix+containers (containers solved the "having to build for multiple platforms" issue)?
Ah my apologies - We use Bazel to build our services, and the output artifacts were then pulled into Nix and deployed as Nix packages. Bazel has _excellent_ support taking the same application code and creating Docker images from them (https://github.com/bazelbuild/rules_docker#language-rules), and the tools available for deploying containers is orders of magnitude more feature-full and higher quality than what you get with Nix today. So we no longer have Nix anywhere in our pipeline, and all of our artifacts are now deployed inside containers.
However, it can be quite cumbersome to get a Nix expression to the point where it builds reliably for something that takes multiple steps like a Rails app, especially if you're building on macOS and deploying to Linux. It's come a _long_ way in recent years, but with Enterprise customers now embracing containerization we migrated everything to that and haven't looked back.