I've picked up the Factions expansion for Alien Frontiers, and have played a single game with it. I ended up with the Proxima Centauri Scholars, which have the ability to place the various Fields once per turn. The fields (positron field, isolation field, isolation field) are only rarely placed in a normal game; they're moderately powerful, and give more options to attack other players, but they require discarding Alien Tech cards. Adding the Scholars to the game makes it much more directly aggressive, with the Scholars doing the vast majority of the attacking. This is a very sharp change in the way the game normally goes; one of the players at my game became rather dejected because I had the option to stomp on anyone I wanted, at a moment's notice. (The Isolation Field is extremely effective versus Burroughs Desert, and still pretty darn powerful elsewhere.)
I've decided to remove the Scholars from future Alien Frontiers games, because it very sharply changes the way that the game plays out, making the game antagonistic in an unbalanced way: one player has a nigh-monopoly on one of the better aggressive tactics. What other Factions cause such large changes in how the game is played, and why?