So NFP builds better marriages?

Yes, but not automatically.   Couples rarely begin to practice Natural Family Planning out of a desire to improve their marital relationship.   However, if they are going to practice NFP harmoniously, they soon find that they have to communicate more fully and creatively with each other.  Couples do not ignore each other at times when they choose to avoid sexual relations, rather they develop non-genital ways of expressing their love and affection, which is the art of marital courtship.