I have no problem venting (not yelling) at costumer service, but i always start by telling the person there that i know its not his/her fault before i rant.
Never abuse the person you are complaining to.In the very least, its counter productive. You need them to want to help you.
I actually said this when I worked at a library. I allowed people to vent, but I wasnt a punching bag. Some people went straight for the jugular and got personal.
“Question for you. How did you get in my house?”