Are you using channels or flakes? Channels are non-deterministic as your nixpkgs essentially changes completely. Flakes always are deterministic as long as your binary cache is valid or the network hasn't bitrotten, or you managed to escape the nix sandbox somehow (or used --impure). Guix is much newer than nix so it has less pain points, but IIRC guix has no flakes equivalent which is kind of deal breaker to me personally.

Of course nix also has one quite big issue, and that it is input addressed rather than content addressed, but its moving towards content addressed model which will reduce rebuilds and resource use https://nixos.wiki/wiki/Ca-derivations