Yes, if you choose to cook your meatballs by simmering in sauce it is possible to overcook them even while simmering. Overcooked meatballs may become soggy and fall apart. You want to make sure you remove the meatballs from the heat when they are fork tender and cooked through. It is possible to cook meatballs in a slow cooker. This may result in very soft meatballs but should not be a problem if you stick to the recommended cooking time.