Tigris & Euphrates is a very good game but the luck factor is FAR more prevalent than its fans will (usually) admit. I'm not talking about bad luck when you don't draw the tiles you need, that's one of the things I like about the game -- how do you acquire green victory points when you don't draw many green tiles?
Rather, it's the wild swings of fortune that can occur when a player wins a battle due to a fortunate (and unlikely) tile draw. It's Chaos Theory at it's finest: Your opponent has 3 blue tiles? You win the battle, gain victory points, and are in a position of strength. She has 4 blue tiles? You just wasted a turn, gave her many victory points, strengthened her position, and improved her tile holdings! Such swings of fortune are brutal and happen far too often.
For a lesser game, this flaw would be fatal but since I do enjoy the challenge of achieving a good score, I can overlook such a high luck factor in an otherwise brain-burning game.