I have been on the receiving end of a lot of verbal and emotional abuse and dishonesty from two people who were not mentally healthy, a partner who struggled with depression and anxiety, and a metamour who struggled with anxiety. (The two relationships were not connected in any way.) The partner was receiving help and was medicated, although our troubles started around when he tapered off his meds quite a bit. The metamour had a few therapy sessions late in the game, after her behavior turned abusive, but was not getting any effective help that I knew of.
I would not knowingly enter into a relationship with either a partner who had issues like these, or a partner who had a primary partner who did. I know firsthand the havoc and pain that can be caused by people who are not emotionally healthy. I want only happy, emotionally healthy people in my life if possible. If I was better at walking away from relationships once they turned toxic, then I might feel differently, but that is a particular problem of mine, I feel committed to people I care about and I tend to hang in there and try to work on and heal the relationship. And sometimes, it can't be healed because one of the people involved is just too damaged.
No offense is intended here to people who struggle with psychological issues of their own or have loved ones who do. This is just a personal choice I've made for myself.