Hard to be a Star Empire if you only have one star.
How so? You have a star, you have an empire, what more do you need?
What might be more difficult is having glorious battles (like the Centurion and the Commander had in the past) when there's nobody to fight but fellow Romulans. But then again, the episode establishes both the violent and infighting nature of the Romulans and
their Vulcanoid heritage that would give them lifespans long enough to allow for a military career before
the RNZ was slapped around the Star Empire...
The seperation hasn't been long enough for there to be anything but cultural differences between Vulcans and Romulans.
Unless there were biological differences before the separation already - possibly serving as the excuse for the separation itself. After all, how do you recognize a Space Nazi, evicted from Earth in 1945 and now returning with a vengeance? By his dashing Aryan looks, of course!