Firstly, regarding curicullum. Many home-schooled students are home-schooled because their parents do not agree with what is being taught in schools, particularly evolutionary theory. These parents do not want their children taught evolutionary theory, and so take them out of schools to prevent that happening. Instead, they are taught creationism. In my opinion, this is harmful to the child. Society has a responsibility to children to give them the best education it can; teaching them a falsified belief is not doing that.
Secondly, regarding socialisation. One of the most important purposes of the twelve or so years of schooling (before university level) is to make children socialise, work, learn and interact with others, including those from diverse backgrounds. In the process, they come across others with widely varying beliefs, and learn to interact with them. This is, of course, excellent preparation for university and the workforce, where they will be forced to have this sort of interaction, like it or not. Home schooling denies children this facet. I realise that many home-schooled children have interaction with other children, through clubs, sporting groups and so forth, but in my opinion none of these can take the place of constant interaction for thirty or so hours a week, every year.