Divorces vary so much, it is difficult to generalize. Some are reasonable, with the parties working to do the best they can for their children. The couple may have grown up and aged out of love and out of their marriage. But there is no real animosity, and they are willing to cooperate in raising their children.
But there is the other side of the coin. Their matrimony has turned into acrimony, and every interaction is full of tension and conflict. Not only are such situations bad for the parents, with all of the additional stress, and it is typically this element of the divorce process that can harm children.
It is not the actual separation, but the tension, conflict and stress that accompanies a marriage and divorce where the relationship between the parents has deteriorated to constant, bitter fighting that makes divorce problematic.