Advantages:
1) parents can devote all their time to their child
They do not need to split their attention, and can better interact with their only child. All the free time they have can be spent with that child, so the child gets more guidance from and bonding time with their parents.
2) parents can be less financially burdened with just one child
They do not need to provide for so many people; they can just focus on their child's needs.
Disadvantages:
1) the child will be lonely without the company of siblings
He has to spend his whole life without a single sibling, and although he can have many friends, their company will never be the same as one of a sibling.
2) the child may become a spoiled and pampered brat
With the full attention of his parents showered on him, the child may start to develop bad mindsets. For example, he may think that everyone should pay attention to him like his parents do, and that the world orbits around him. He may take everything he has for granted. Nobody will like him and accept him into the society.