The "risk" with that is that someone who is particularly sensitive to caffeine and who is inexperienced with alcohol will keep drinking past their limits because the caffeine hides some of the feelings of drunkenness from them.

Guinness tends to be extremely weak, so one is more likely to get sick and/or struggle to even drink the whole thing due to how rich and foul tasting the whole thing would be, meaning it would be a challenge to even get one drink down in an hour with that.