I have finished 6/8 of the class stories.
I would rank them thusly:
IA > SW > Smuggler > SI > JK > JC.
I feel almost exactly the opposite regarding the Jedi Consular and Imperial Agent. The Agent story is completely absurd and proves the OP's point. I don't believe the Sith Empire could conquer anything. All three Acts, the Empire characters are following these mostly myopic and personal stories while the Republic is kicking their butts off planet after planet.
The only major world they manage to capture after Chapter 3 is Taris. They lost Balmorra and Corellia, no matter which sides' stories you're playing (Republic Balmorra is after the Empire victory in their version). And losing the weapons factories and the shipyards means they've effectively lost the war. However, as I said, I don't believe this society could have ever achieved much of anything.
One example, failure and the Empire's response to it. We know that failure and struggle are absolutely essential to long-term success. If you always succeed at everything you never learn from any mistakes. The Empire executes anyone who makes a mistake and can't finish a mission. That's so cartoonishly silly that I have a hard time understanding how anyone takes that side seriously.