If the information is about a subjective thing keep quiet. After all different strokes for different folks.
Health (physical, mental, emotional) issues can be assumed were checked and okayed by the respective party involved. If you have new information you have no right to blab.
But, information regarding criminal activities involving the future spouse or their family MUST be revealed. Criminal behaviors can entrap and ensnare innocent people who with advance knowledge can back out of the engagement.
Some families and people are simply not marriage eligible.