I think games are the biggest risk. My son’s school has a requirement for an ipad for junior students and a laptop for senior students. On the surface the portability is seen as a plus but in practice it makes it too easy for the kids to goof off when they are supposed to be studying.
My son has a serious gaming addition, as well as spending too much time networking with friends. The tablets, laptops and smart phones make it almost impossible for us to regulate it.