Joint meaning?
If you have 2 parents that are honestly committed to the wellbeing of the children, it can work well. Generally it requires a cooperative relationship between the parents. In other situations it isn't even a healthy option. Divorce often involves one or both individuals who are dysfunctional in some way to begin with. Often legal joint custody between these types of people does little more than reduce the child to a psychological "chew toy."
I know the thinking, at one time anyway, was that it was important for a child's stability for there to be one parent who had the final word in disputes. With reasonable people there is a lot of room for flexibility in a sole custody arrangement. Of course, the key here is "reasonable" people. All too often those involved in custody battles are neither reasonable nor putting the child's best interest first.
I have seen joint custody work well, and I have seen it be a disaster, with the children having no sense of permanence with either parent. What you end up with is the perpetual involvement of the state and 2 or more families with their lives completely under their control, which, as we should know from any number of other situations by now, isn't looking out for any body's interests BUT the state's.