I'm very familiar with diabetes. My late stepdad had it, my grandmother has it as does my half sister on my dad's side and as did my late paternal grandmom.
For me, in tossing these illness out there together in terms of when to disclose to a romantic partner seemed ok because diabetes when properly treating isn't something that needs disclosure right away. I felt that each after getting to know one another, a delay would seem justifiable in the sense that, hey look, watch how well I've taken care of myself, but it's important to know going forward, a relapse could occur with bipolar-provided it's being treated and monitored or hey look it's important to know that there could be a high or low sugar moment, here's what to do.

With MS, it's stress that can be a major factor. Eventually, as a relationship evolves, recognizing that stress or colds or no clear cause at all can be trigger enough for a flare up. For me, better communication reduces my stress immensely.

But I see you are saying about untreated mental illness and how damaging the results are.

"Investigate your hidden assumptions."-Cornel West