If the Federation was more nuanced, it could work.
I don't know what federation was ever meant to be, but here's what it's now: a corrupt sprawling bureaucracy of galactic oppression.
If the blurb is closer to this than the current
positive spin, people would be less tempted to join it by default.
They don't need to be complete bad guys, but a corrupt, ponderous bureaucracy based on what should have been, ideally, a nice political model, will probably talk to many without feeling like the good guys.
Dynasty feels like those unrepentant bad guys we love to hate (and play), and should definitely keep playing that angle.
Good old evil ancient conspiracies are always fun!
Empire is facing an old and interesting dilemma.
On one hand, empires are very romantic. I mean look at those monuments! Look at those flags! Look at those uniforms! Look at this badass military administration!
On the other hand, they have, with very few exception, always been historically the worst jerks around - by nature. An empire is, fundamentally, a wealth pump: a political system that exploits the periphery to enrich the core, and that play every level of the system against each-other (each group trying to get a higher place in the pyramid) in order to prevent them from uniting. Which often results into long-standing hatreds long after the empire is gone. Also, it is fundamentally unstable: the core always wants more, which requires a harsher exploitation of the peripheries, at some point beyond what is sustainable. As such, it ends up running dry, and crumbles.
Of course, the Empire pretends to bring peace, order and prosperity, its But those always finds itself moving from the peripheries to the core, strangely enough.
The Roman Empire is the archetypal example of both.
As fictional examples go, the biggest one is probably the Galactic Empire from Star Wars.
One of those very few exceptions was the Achaemenid Empire, which is the only lasting one I can think of to actually live to standard imperial propaganda: an efficient, rather fair administration, peace, economic development and infrastructures ending up profiting to all... It was, for its time, one of the nicest places to live. Even more impressive as it nearly covered half the world population.
The fell mostly because their didn't reform as the situation changed - when a system has always worked, after all, it can be hard to fix it.
But I digress.
The point is, by playing on both the romantic appeal and the sinister reality of what an empire is, they can be made to work at any desired nuance (ideally leaving it open to interpretation) from the dream of Cyrus the Great, to the which example to choose without offending anyone. Not this one, not that one, not this one either, certainly not this one... well, let's go for the cliché then