FAs have to make a judgement call and they have no clue or should they be expected to know if and/or when a person could become problematic from excessive drinking. They may not always handle it in the best manner but it's not something they probably want to do or find it easy to do.

An airplane is not a college party kegger. No one needs to drink like prohibition is coming, even if it's complimentary. I personally would be horrified if a FA "had to cut me off." Now the real problem and where there should be limits are the airport bars. That's where the in flight alcohol problems originate but there's too much profit in booze.