They are tricky. So, so tricky.
But I do want to tell one story:
A boy I knew had had his heart broken by a girl he really liked. And his family, consequently, talked kind of badly about her. She wasn't good enough for him, she treated him badly, she needed to grow up and stop being so superficial and insecure, if they ever saw her again, they'd somethingsomething. Some of this was in jest--they are loving people--but some of it was said earnestly, in their hurt for their brother/son, whom they loved.
Most of the times I heard them say these things to my friend, he would try to deflect it. He would say, "No, no, it's okay. It's fine. I did x, y, or z, and that wasn't good."
Once, on one particular day, she came up in the conversation, and his brother made some comment about her and how she was a bad person, and my friend looked his brother in the eye and said, firmly, kindly, "She's doing the best she can."
I decided then that that was one of the most Christ-like things I had ever heard a person say. And I knew that if he and I ever broke up (yes, we were then dating), that he would treat me well, subsequently, and that I would not have to worry about how he would talk to people about me, including the ones closest to him, including his family. We later broke up, and he has treated me well, and I have not worried.
And I have tried--tried to remember to try--to follow his example in all of the situations I can. Even though, I want to acknowledge, that it may not always be true. We may be 100% confident that our ex did not try as hard as he/she could; still, still, I think we should treat them as though they had. I don't know. I'm not sure I can defend it, except that any other idea makes me feel sad inside. And this story makes me feel happy and full of grace and truth.