I know some darn good, loving, loyal commited couples who aren't officially married and some unfaithful, insecure, unstable couples who are. And everything inbetween. What I really admire, though, married or not, are the couples who have survived hard times (which are inevitable) and grew into loving, stable, trusting, commited, permanent family to each other. I see them holding hands in their 80's, proud to be together.
I think everything has its advantages and disadvantages. Living together without marriage may be less fraught with demands and expectations. It can vary from roommates with or without friendship to lovers and mates and still feel acceptable. No one has to feel "stuck." There's a renewal of choice to be together just by virtue of staying together, not because of a legal bond, but because each one wants it.
With marriage comes more expectation of support, sacrifice, consistently meeting each others' needs and teamwork. It may be easier to build resentment in marriage when the expectations aren't met. Conversely, the legal bond may make it easier to recuperate from division and feel unified even when independence and separations are built into or happen in the relationship. That's when things work, though, because, otherwise, the sense of loss and shame when the relationship fails seems even worse than a non-marital break-up.
To be successful, long marriages or relationships better learn from mistakes and may be like different relationships in different eras of the relationship. Serial marriages or relationships better learn from the mistakes made in previous relationships to work. Sometimes, the newer relationships aren't better but they are nurtured more carefully for fear of failure. People learn to be more tolerant and accepting and learn that it may be better to be married than to be "right" to a certain point.