We've coslept with both our children and don't have sleep problems.
It depends how you define problems. But if you asked my CIO acquaintances, (if they knew the reality of our sleeping arrangements, that is LOL), they would say that we have enormous sleep problems in our house. They expect to put their children in a bed or crib at 7.30, say goodnight, and not have any more parenting duties until the morning.
The fact that I parent my two girls to sleep, then roll over in my sleep and nurse my toddler whenever she wakes, would, to them, be the most almighty type of sleep disorder.
But I look at them, and think that their children have terrible sleeping problems. They are disconnected from their parents. They are isolated for twelve hours a day from human contact. They suffered physiological and emotional damage from being left to CIO as newborns. They are being set up for sleep and attachment disorders throughout life.
So, my point is that 'problem' depends on how you look at it. If you want a form of convenience parenting that fits in with the TV schedule and a social life, then to co-sleep will be perceived as a problem. If you want to parent the way that nature intended, then the cio brigade and their children have a problem.
Just another perspective. But the cio people don't post saying "Oh, I'm really worried about the harm that cio might have done my child. Can anyone reassure me?" Instead, they sit down with their newspaper or for their adult time, without a second thought. An acquaintance recently told me very assertively that crying it out 'does no harm'. He knows, because three of his kids have been through it, and they 'learned' after a few nights. In fact, research shows that it does do physiological harm, and I was about to point this out, but then thought, what's the point? The culture is such that the fact that something 'works' and is convenient makes it 'right' and harmless. No need for any depth of thought or consideration. Sad, imo.