< But remember anyone can get I'll - it's the same kind of issues you'd be facing if you were with someone with diabetes or heart disease - it might rear its head, it might not.>
But it is not really the same. You can have a better idea of what is potentially ahead of you with some physical conditions. Even if someone is substantially physically disabled the sick person can still be there for the healthy person emotionally (the most important part of a relationship). There would normally be a stable core personality in the person with a physical health condition, which may not be the case with people with certain MH conditons.
But with some MH conditions the person can have a lot of relapses which can be very scary/draining (it does depend on the condition and how badly the person has it). The personality of the person with the MH condition might change dramatically some of the time, and sometimes very quickly. Medication can cause problems such as emotional deadening. Or sometimes the meds just don't work well for the condition, or because of side effects (or the sick person deciding they are now "cured") people can come off meds and relapse (personally this would be one of my major worries in this situation).
I knew someone who married someone with schizophrenia and he was basically abusive and unavailable emotionally (admitedly she was not very nice either!). Though this was years ago and medication could be better now.
I would not be happy that they kept this to themselves for so long either, even if it is understandable. He really should have brought this up earlier.
Personally I would probably bolt because of the experience the person I knew had with a schizophrenic partner, but it would probably be more sensible if you like him you could find out more and then make a decision. Find out especially if he has ever come off his meds and relapsed as this can be a big problem. You probably cannot fully trust his family to be honest (as they might be so happy for him that they sugar coat things in order not to scare you off, or they might be exhasuted from it all if it was very bad, and be happy that someone else is around to be a carer) but maybe you will still find out things from them.
You could maybe see about going with him to his next psychiatry appointment? You could have a chat together then. There will be confidentiality issues for the psychiatrist (they won't be able to tell you anything about him without his permission) but if you ask a question that doesn't get answered you will know what to dig a little deeper about.
Personally I think it is ok to have misgivings, and you should not feel pressurised into keeping seeing this person because you are worried about appearing prejudiced. Yes anything could happen to anyone, and anyone's partner, into the future, but it doesn't mean one should walk into a potential nightmare either or ignore ones gut instinct, red flags etc.