Hi. When you say "unforgivable" I immediately think of something like cheating. If you heard a couple broke up because of cheating, most people would understand how that would cause the end of a marriage/relationship.
I have successfully worked with women who have been determined to work through it and have regained trust and a flourishing relationship.
In terms of ranking marriage problems that I help with, giving up or a lack of belief/willpower in making it work. Assuming that there was nothing "unforgivable" that occurred, life happens and the relationship gets lost in the day to day things. Stress in outside circumstances like work, housework, family etc strain a relationship that is barely there. Relationships that last a long time have two individuals that go through many seasons of life, I help to find how the season they're in can be brought back together. Who have to keep finding ways to love eachother through the changes.
In terms of the top thing, communication is often the first thing that disappears, real communication, trusting your partner with your needs and feelings and having faith in them to be able to understand. A couple can say words and manage a timetable or to do list together, but a couple thats struggling is often a couple that doesn't believe they can keep sharing what's within them.
I'm hoping this answers your question.