Predictability and stability help children develop confidence and the ability to take reasonable risks. Judith Wallerstein studied families who were contesting custody and whose shared custody arrangements were, to some extent, involuntary. She reported on her concern that the frequent transitions under joint custody can exacerbate the divorced child's fear of abandonment. It's because the child feels safe nowhere. Conflict is bad for children, and if you put them more in the middle, it's bad.
Expenses are greater in maintaining two full residences.
There is duplication in need for clothing, furniture, and other necessities. This at least partially relates to the long-term financial strain. Fathers are sometimes unprepared for the actual responsibilities of shared custody. Despite the influence of the feminist movement, the reality is that most women are still the primary caretakers of children, even when they work full-time. Some fathers grow easily into an expanded role, others have more difficulty.