Terran Empire/Mirror Universe seems a little unbalanced. Specifically:
1. You start with a massive rebellion (3K+ fleet power) a year in. I don't see how you can realistically counter that - gaining the Defiant gives you an extra 600. Wow, big deal.
2. As with the primary universe, seems a little lore-light to not start with a bigger fleet and NX-class ships.
3. Slavery doesn't work properly. Every slave planet rebels.
4. Charon-class ships cost an awful lot, and don't do anything. You spend 3000+ alloy and components for 1.4K? Do they do anything else? They're labelled as planet-killers but they don't seem to have that functionality.