Based on my very non scientific observations of dubs that I have met I think there may be a higher percentage of dubs with mental illness than the rest of the population. These numbers may be skewed by
1. Those with mental illness may be more attracted to the prepackaged friends that the Borg offers as well as the promise of the paradise fixing their problems.
2 Born ins without any mental illness may be more likely to leave the Borg. They may be quicker to see the cult for what it really is.