A metamour you trust is a good thing.
I don't have a lot of experience with poly at this point, but the one huge horrible thing that has happened to me was having a hateful metamour. I don't even think she meant to be hateful, but it developed pretty strong, pretty fast, and ended a really great relationship.
Now my husband is dating one of my close friends. I think it's great because I don't have to worry that she will turn all bloody mary on me and try to sabatoge our relationship. I don't have to worry that this random person is getting a lot of private details and judging them, because she's my friend. She has a few years of knowing me and won't immediately judge things wrong, cause a problem in my relationship, or send me weird angry emails. I think close friends are the way to go. Just a thought.