> And if you do have more platforms, why not just check in one node_modules-directory for each?

Now you have to sync it or risk running into unreproducible build failures. Also, if you update the binary dependencies on say, macOS, then you still need some x86-64 Linux to build the dependency.

Not saying it is not possible but without a proper process (e.g. a build server being the only place that updates dependencies) this is going to be painful.