For one, I also have family members who are very alcoholic and 'you'd never know it' either, i.e. fairly highly functional.

The effects of caffeine and nicotine are mild, and mostly physical. Especially with nicotine, it's a 'maintenance addiction' in that smoking in addicts is mostly to get to 'stay normal' because the body has adjusted dopamine receptors. A little bit similar with caffeine in that we build a tolerance.

I don't think it's a stretch to consider that there might be some issues with a good chunk of society walking around stoned all day. I'd put 'drunk' in a similar category.

The people who I know who are like this are themselves concerned but they can't seem to change their habits. It's not just a matter of 'physical/mental' state - they are basically different people in this mode and they know it.