One of the greatest consequences of the terrible advice to be nice is the nullification of some of the most important and worthwhile intended effects of close relationships. Relationships with spouses, children, parents, and dear friends are meant to provide deep meaning and companionship. None of these intended outcomes can occur with relationships sterilized by niceness.
Intimacy is a measure of the depth of shared reality. It is a measure of shared awareness and co-existence of thoughts, feelings, and actions. The meaning that can be shared between any two people is limited by the intimacy shared between them. The limits of intimacy you experience with others in life probably indicates a similar limit in your intimacy with God, for all the same reasons.
Intimacy cannot be attained or maintained without conflict. To be a true friend, and especially a loyal spouse, requires you to do all you can to help each other be the best you can be, which certainly requires you to correct each other, especially telling each other what no one else can see, what no one else cares enough to tell them, and what they least want to hear.
Too many suffer from relationship sterilized by an unspoken quidpro-quo: you will not criticize them if they will not criticize you. To ignore differences is to lose whatever unity you had before as well as to miss out on the opportunity for greater intimacy through reconciling to more of revealed reality together. Sameness through intentional ignorance is not unity. What is the point of having a relationship with someone when it is only maintainable through a lengthy list of things you can’t talk about or do together, or parts of you that the other person doesn’t want to experience?
Intimacy is maintained and grown when two people respond by making the changes necessary to remain reconciled in light of a greater revelation of reality. Life will reveal new differences in thoughts, feelings, or actions between you and any other person. A greater revelation of reality acts as a catalyst, not just to reveal previously hidden differences, but for one or both people to improve their thoughts, feelings, or actions beyond prior hidden limits. Intimacy is increased through discovering and resolving differences, and decreased through ignoring and maintaining them. To the extent that these differences cannot co-exist, it is incumbent upon you to address them.