I've heard some people complain about how the Cardassians were just lamer versions of the Romulans.
Nah, I think the Cardassians are distinctive enough to deserve credit as more than just a poor man's Romulans. Yes, both cultures were militaristic, but I think the main distinguishing characteristic between the two is how much more formal, restrained, dignified, and cordial the Romulans are.
In contrast, the Cardassians are more likely to mouth off and express themselves without holding back their emotions so much. I think the Romulans like to play things closer to the chest generally (although Tomalok seems to be a bit of an exception with how emotive he sometimes was) and that makes sense since they're related to Vulcans.
I don't believe the Cardassian episodes could have been easily interchangable with Romulan episodes. Just look how much the tone of "In the Pale Moonlight"
and the behaviour of Romulans in that episode differs from both in Cardassian episodes. The Romulans are so much more quietly menacing and smug than Dukat, Damar, or Garak would be.