It's not that installing Nix is impossible on macOS, it's just that it has some hard-to-ignore limitations now.

1. Having to create a volume when a plain old directory should suffice is insane. It's creating a hassle for no good reason for users.

2. /nix would be unencrypted by default if kept in a separate volume. There's also the problem of how to unlock it upon boot.

3. Login items is a very common use case so not supporting it would be problematic for many users.

4. Unreliable sleep is an even bigger problem.