Disclaimer: I own nothing. Descendants belongs to Disney.
Everyone on the Isle has had experience with mental illness and/or addiction. Plenty of the villains placed there had mental illnesses before being imprisoned on the Isle and got exasperated by the poor conditions. Many other villains became mentally ill after being imprisoned on the Isle.
Maleficent shows symptoms of Borderline Personality Disorder, which have only been exacerbated since being imprisoned on the Isle. Jafar shows symptoms of Antisocial Personality Disorder. Since being imprisoned, he's become very paranoid and keeps to himself. Cruella shows signs of Dissociative Identity Disorder, but as she has never been diagnosed, we may never know what she truly has. Since being on the Isle, she started struggling more than she was before. Due to their untreated mental illness, their children end up suffering.
VKs themselves are predisposed to mental illnesses because of their parents. Certain mental disorders tend to run in families. Their stressful living environment also makes them predisposed to mental illness.
All people on the Isle suffer because of mental illness. Quinn, daughter of the Queen of Hearts, often gets neglected due to her mother's untreated bipolar disorder. Jay, the son of Jafar, is forced to steal from his father's shop and get supplies for them, as Jafar is scared to leave his house. Evie, daughter of the Evil Queen, suffers from an eating disorder due to her mother's pressure that she should be pretty and thin.
Many people on the Isle suffer from some sort of eating disorder, whether it comes from pressure from parents to stay thin, or from themselves because they don't deserve to eat. They are hard to manage, and many people die from them.
Many people on the Isle also have anxiety and/or depression. Both villains and VKs have lots of anxiety from their environment. They never know what is going to come next. Depression is also prevalent for both groups, as their lives are complicated and, for some, not what they used to be.
Addiction is also prevalent on the Isle. Many people tend to self-medicate with alcohol or any drugs they can get their hands on, and due to unsafe drinking water on the Isle, alcohol is a common drink for people of all ages. Drugs and alcohol get to the Isle from what the Aurdianians throw out; they aren't common, but do appear. Some people on the Isle brew their alcohol with things they find on the barge.
Many of the villains are alcoholics. Captain Hook and Gaston are both alcoholics; due to their alcoholism, they neglect their children and can get violent. They both care more about alcohol than they do their children. Harriet Hook is afraid that her father will die from liver failure due to his alcoholism, and she will have more responsibilities than she already does.