“The dating situation is artificial, but if you see someone enough times you’ll get some sort of idea about how they think and react.”
Others commented about some particular number of dates, or whether those who date more vs. less end up in beis din.
Aside from there being virtually zero statistics on this, I question anyone reporting this based on anecdotal data. I do not believe there is anything at all to do with more or less dating. I do believe that mental illness is one of many factors that can become a problem within the relationship, and that the failure to discover or disclose it before is problematic.
But if we recall that the dating period is completely artificial, as noted in the comment from Midwest2, the amount of that contact does not matter at all. It is typical that when one is seeking to establish a relationship with another, that they put on their best face, both in physical presentation and in their demeanor. These two dating people are hardly the real people, and this prevents them from seeing each other under pressure, dealing with negativity, disagreement, etc., all those things that need to be part of managing a marriage. How is he/she when they are hungry, tired, or in a bad mood? That will never be discovered on a date, not the first, not even the tenth.
Attributing success vs. failure in marriage to these numerical factors is erroneous. It completely misses the core issues that determine whether the marriage has potential. The older ones among us can likely remember grandparents or great grandparents who barely dated, if at all, who enjoyed beautiful marriages for many decades. The parsha about what makes marriages fail or succeed is vast, and efforts to present any single factor are futile. The question here is about the secrecy of mental illness. Yes, it can make a huge difference. And there may well be certain people that should not get married altogether. I know that is a terrible thing to say. But not possessing the basic skills to manage a relationship makes such efforts doomed. People need to spend their lives being happy, and this requires making the other person happy, too.