Being a "functioning alcoholic" means you are i) addicted to alcohol and thus likely to be drinking much more than is healthy. Long term heavy drinking is associated with a bunch of health risks (the obvious being cirrhosis and the less obvious being some cancers).

I guess it depends how you define "alcoholic".

"functional" is a low bar - impairment in memory and cognition creeps up on you, and it's possible to boil like a frog until the onset of liver failure and permanent neurological damage (eg delirium tremens) dispels that illusion.