From the preview:
At its best, the Imperial Assault campaign game rockets from action to intrigue and back to action. While one player controls the Empire and all its seemingly inexhaustible resources, the other players become Rebel heroes, desperately fighting to complete key mission objectives even as Imperial stormtroopers are firing at them and their time is always ticking down.
And while each Rebel success advances the heroes toward their overall goal, and each Imperial success allows the Empire to more fully impose its tyrannical will on the galaxy, your campaign isn't truly over until the last beat of the finale's climactic action. Up until it's all finally resolved, there's a chance either side can win the larger battle and help steer the fates of countless millions.
All of this is still true in the new campaign introduced with the Heart of the Empire expansion. In fact, all the action and intrigue isn't just maintained; it's magnified.