Brawn (AI on suicidal) might not always beat brains, but only because there is no point to having an unbeatable difficulty level.
Also you will find with multiplayer, that fairly standard tacktics will develop among elite players whereby anyone who dares try anything different would quickly loose, kinda like playing 'tic tac toe'. That will become monotonous and booring.
The AI is always beatable largely because it has inherent programming flaws in it's logic and tactics which are constantly repeated. once aplayer has noticed those flaws, he can easily win. Tic tac toe, on the other hand is a game with limited possibilities where one can achieve a win or draw everytime without effort. A win requires stupidity or inexperience on the part of your oponent.
In Multiplayer GALCIV There would be no AI to apply the same tactics each game. All players would be free to play creatively. If the "war rush" tactic is eliminated or countered effectively then I bevieve that this game would go on for hours. Since the civs could no longer trade techs under the table as the AI controlled Civs presently do, tech advancement would be slower as a result.
Like all other multiplyer games, the weak point will be in the civs ability to expand its empire and improve its industry and resources.
But the logistics of multiplayer GALCIV may prove difficult to overcome. Waiting for the other players to make their move will be exponentially more boring as the number of players increase. The game would have to send you an email when your turn comes! Although you could be designing ships and planning stratergy while you are waiting, once your opponents make a move you may have to re-evaluate your plans.
What happens if a player quits part way into a 3 month game?