I just want to weigh in briefly here to state that I think that the "Hard" AI is often more difficult than the "Brutal" AI, particularly in spread-out maps.
From my experience, the Hards tend to attack with small numbers of dudes all over the place, staying out of range of defensive structures and harassing any unprotected mexes, while Brutals wait a little longer before they make their first attacks and strike with a stronger force that's more likely to win if intercepted.
Thing is, many players don't have units online early enough to counter the early aggression of a Hard, and on spread-out maps, it's a real chore to keep your mexes online.
I'm not complaining that the AIs are bar or anything; it may simply be that they're complementary (one might be labeled "Harasser" while the other is "Builder" or something).