My experiences with mentally ill friends have not been good. Obviously not all MH people are like this, but many are, and it's hard to tell in advance.
One decided to go off her meds and yell at me constantly, then apologise, then yell, then apologise, so I broke it off.
One has suicidal ideation, and keeps trying to kill herself despite being on meds. I have distanced myself, because I am not qualified to be a rehab centre, and it's frankly draining to constantly get messages about how she wants to kill herself.
One has severe health anxiety, and constantly needs me to take her to A and E at 3 am, because she thinks she is having a heart attack. I am still her friend, but I have politely refused to take her, and luckily she now lives in a different city. I now don't travel with her, and only meet for a meal.
All these women are successful professional women presenting as "fine", have access to treatment, but are still quite hard work behind closed doors, even when I only see them once a month or so. I don't want someone I see daily to be hard work.
I have and will date diabetics though. In fact, DH is one, but he controls his diabetes, and is not nearly as hard work.