system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek] Why aren't Romulans and Cardassians more worried about the Borg threat?

These two empires are almost paranoid about their own security, but they seem to devote more time and resources to sabotage the Federation a bit or deal with the small threat of the Maquis than to learn and defend against the Borg.

Is there any expedition to try to reach their quadrant to research about them?

Any secret weapons that they were developing in secrecy?
gpt
They are (especially the Romulans). However, they obviously aren't going around and spreading the news to every Federation ship and person they come across, so that's why we don't ever hear about it.

If you are wondering why they never seem to join efforts to stop Borg incursions into Federation space, it's because why would they?  They probably see the Borg attacks on the Federation as the Borg doing their job for them.