Congeners cause hangover - byproducts of fermentation in some alcohol. Dark alcohols have more of it than clear white. Now, mixing different congeners ( in mixed drinks) and combining the various impurities can result in particularly severe hangover symptoms
Drinking beer and liquor cause hangover too. Is the carbonation that speeds up the absorption of alcohol.
You can see often "cheap" drunks drinking beer and liquor for the fast "effect" of getting drunk.
Alcohol metabolism is even more toxic than alcohol itself - it is stored in liver and builds up in the body. Several chemical processes happen that prevent you from sleeping well the night after heavy drinking, cause stomach pain and nausea. The stomach pain tell your brain that you need to vomit and get rid of the toxins. It usually helps and you will feel better almost instantly. But stomach irritation often cause other unpleasant symptoms such as diarrhea and lack of appetite.