Alcohol is what stresses the liver, and even scars it when abused (cirrhosis), as well as increase the chance of anemia, cancer, cardiovascular disease, dementia, depression, gout, pancreatitis, nerve damage, diabetes, shrinking frontal lobe, infertility, and problems associated with dehydration. Like all other drugs, moderation must be exercised when consuming alcohol (alcohol actually has several health benefits when consumed in moderation).
As for a hangover, it isn't from the additives but definitely the alcohol, and while it isn't known exactly what causes them and why, it seems possible the culprit is a combination of things including stress to the body, the body trying to re-regulate and correct its functioning, irritation to the digestive tract, acetaldehyde build up (a byproduct of metabolizing alcohol), and large amounts of cytokines being released into the body (something that is normally released in small amounts to help fight off infections).