Don't air out your dirty laundry, my partner always told me.
And the more transparently I hear people talk about their relationships, the more I realize that there will always be objective issues with every relationship. That's why it's dangerous to vent. Your friends and family will often, in a biased and potentially unhelpful manner, be on your side, and your partner will feel uncomfortable, thinking that they now have a bad reputation with those close to you.
I think relationships are great because it's an opportunity to enter a contract, formal or informal, of mutual happiness, loyalty, and unconditional support. And that commitment to unconditional support is definitely scary. It means putting aside, or deferring, our own common sense (and silencing criticism) to enter the headspace of somebody else and empathizing with them, considering their own biases, values, preferences, and dreams.
In a relationship, you can't always act on what you think is objectively best for them. Their modus operandi is likely different from yours, and you have to be okay with it, or at least come to a compromise. In a relationship, you'll learn all sorts of quirks that you think may be uncommon social behavior. In other words, you might have to adapt to a new normal and thus realize that everyone really is different in little ways.
So I think airing out dirty laundry, or smack-talking about your partner isn't productive to the well-being and longevity of a relationship. After all, if you can't trust your partner, who else can you trust?