The Romulan Star Empire is pretty much like the Vulcan and Andorian empires were before they were incorporated into the UFP, only the RSE remained seperate and grew seperately. The Romulan Star Empire seems more benign to its own race than the Cardassian Union for example and the standard of living seems more pleasant than in the Klingon Empire, and their ships and military are more efficient as well (one gets the impression their forces were not as militarily burned out as the UFP and Klingon Empire were in the Dominion War).
We have no clear idea how Romulans treat conquered people, but judging from "Birthright" and Sela, they don't seem to be as racist and vindictive as the Cardassians or Dominion.
The Tal Shiar are perhaps like the French intelligence agency in that the civilian government has no proper control over them, they've cultivated a bad reputation, but don't affect most people. Also the fiasco at the Founder's homeworld earlier on in DS9
did not seem to damage the Romulan's political stability and military strength as badly as it did to the Cardassian's (also the Obsidian Order was rendered virtually defunct, while the Tal Shiar was still extanct and significant much later in DS9