Serious dizzy spell on the outside lane could be a hazard as could serious depression when you are unable to think straight or have suicidal tendencies. Being a GP would you be willing to take the responsibility for such situations or just take the easy route and inform the dvla.A dizzy spell or depression - absolutely would not affect a person's ability to drive. Why do you think they would?
A drink problem... well that depends on whether the person drives while under the influence. If they're sober enough to pass a breath test then I would think they're just as capable as anyone else.