They're a race of brutes who like to throw darts at each other, get into bar fights and basically being very unpleasant. All of which makes them unpredictable and therefore untrustworthy.
I'd rather do business with the Ferengi, at least they have a code of conduct that helps you to understand them and how they operate.
The Breen should also be on the list somewhere. If the Romulans have a saying about never turning your back on one, you kinda know they're not nice people.