Who participates in COB/what are the chances of a bid

Has anyone had luck with COB in recent years? I encouraged someone to rush because I had such a great experience at OU with my sorority and she was literally dropped from 8 sororities after round 1. She’s so pretty, smart and funny that I was shocked by this. She ended up with AOII, AGD and Gamma Phi after round 1. Gamma Phi dropped her after philanthropy. She did not vibe with AOII or AGDso she dropped out of formal rush. I feel bad for encouraging her when it’s been kind of a bad experience for her. Which sororities usually participate in COB? I just don’t want to get her hopes up again if there’s a low chance she will get a bid this way.