Just speaking from my own experience, my son would literally cry and cry and cry and cry. He never stopped. It didn't matter if you went in to soothe him every five minutes, every ten minutes, every fifteen minutes. If I stayed in the room, if I left the room. He started getting so upset that he would become agitated enough to try to break out. It meant he started slamming his head against the crib (with quite a bit of force) to the point that I was terrified of a concussion. One week I decided to see how long he would cry. It was SIX hours in a row and I stopped it, he didn't stop crying himself. For me personally, CIO is controversial because people would just tell me to give it more time or I was doing it wrong. It was never that maybe my baby was different and couldn't handle that yet. And it was never okay for me to give up. My son was clearly experiencing severe psychological stress from CIO and I received so much judgment for refusing to do it after a month. (And I tried it for a month at three different ages in his first year.)

Turns out, my son his terrified of the dark. He's almost three now and has to sleep with four nightlights, the overhead light on, six stuffed animals and it's still a struggle to calm him down to sleep at night.

CIO is a great method, if it works for your child and your family. But not every family and not every baby is the same. To treat it as the end all, be all, only thing that works is frustrating for those who it doesn't work for.