This is a great topic thanks! Something I was always agreeable to was a partner that was a project, that needed help with their MH. What i learned was there's a million miles between "here's my issues, here's what I do about them" and someone who doesn't acknowledge that. So it was a would to an absolute no under any circumstances because the relationship was a disaster, I was horribly hurt and long term I made no impact on them.
I was a "never" about people with kids when I was younger cos I've no clue with them. That definitely changed because I really saw my role would never, ever be parent to the child... step parent is different, you're not there to raise someone's kid for them. That wouldn't be respectful. I'm embarrassed I was so rigid on it when I was younger!