Talking ski here. To see a ski game purely in terms of challenge might be too narrow a view. It's RTStrategy, after all. I think it all boils down to the map, rather than the balance(ie the mod). The AI will always play a linear game; that is, it will always be either winning or losing. It cannot hold ground or manoeuvre its opponents (though Troman's Aivolution came pretty close to that). It has a number of options, though. It attacks the weakest enemies. If is destroys an enemy's asset(s), it will then concentrate its attacks on that enemy. But in all cases, it will withdraw if its losses go above a certain level. It will then not attack again until it has strengthened its forces through research. It will often, though not always, defend its oil - more usually when it is close to its base.
Thus, if there is more than one AI player, the human player may not be the object of any AI's attack, and it is a strategy available to the human player to manipulate the various AIs so that they fight among themselves. Given that WZ plays best with strength against strength - which is how Pumpkin originally envisaged it - the basic challenge facing the human player is to avoid being overwhelmed through the one great advantage available to all the AI players: while you twiddle the mouse as you plan your next move, the AIs will have run through many thousands of decisions. Again, when the AI decides on a tactic, it gets followed though with merciless precision. There is nothing quite as disconcerting as the remorseless drive of heavy tracks through your defenses and base.
So, the human player is always on a knife edge between advancing and defending. You must always strengthen against strengthening enemies, and always move with caution.
What kind of map suits this gameplay best? Test the original Pumpkin maps on v1.10 set to hard. You will find that these maps concentrate on exposure, where the need to take and hold a point will in turn expose you to danger. Rolling Hills is a very good example. You can defend the immediate environs of your base, but pretty soon you find that the area is too small. Once you try to take control of the next circle (see Iluvalar above), you find you run the risk of overstretching your resources. Getting more oil (the next circle) exposes you even further. A good custom map to test is the little four-hander called River Canyon - still one of the best custom maps ever made for WZ. It is hopelessly unbalanced, but the whole game of securing choke points is here raised to as high an art as WZ can offer.
The challenge here lies in the play of strategies. It is often a matter of patience and perseverance. The secret here, I think, is to understand that it is not a question simply of competition - an experienced player will win most games as a matter of course - but something more like play between friends. The more you allow the AIs play their parts, the more enjoyment you get from the game. You should like it when the AIs put up a good fight, build effective defenses, make good use of the research lists, put you under pressure. It's a game, after all.