Lucas even commented himself that the targeting system onboard ST ships would be the great advantage, and the empire didn't finish developement on suncrushers, the post empire did.

If the empire can lose to rebels with limited supplies, how about the undergrounds of a thousand races and empires across the galaxy? The Empire would occupy for a little and then would flee with its tail between its legs.

The Rebels also wouldn't fight the Federation.

ST has great weapons, but it relies on the human drama, and so all those are relegated to other races that pop up from episode to episode.