The rebels have huge armies defending their cities, and these armies, unlike in standard EFS, are mobile and like to counter-attack. Taking a planet now requires a lengthy campaign instead of a few hit and run raids . The tech progression is also much slower, forcing players to conduct low-tech warfare, and making them make important decision as to which tech will be researched next. Spaceships have numerous variants that become available at higher techs , and each ship was designed with a specific role in mind. The ultimate construction project is the Death Star, which requires an entire fleet to destroy, and which can vaporize cities from space with one shot.
All units are a perfect balance between cost and utility.
Try it for yourself and see. The once-great interstellar Roman Empire has fallen. Many of its western systems have declared independent prefectures, each claiming to be the rightful successor to the Emperorship. Others have declared their own kingdoms, while others still have fallen into barbarism. Where the old imperial administration has remained, so has the very powerful Church.