Interesting thread here...
Tribes, Civs or Factions having a different feel in game play is basically accomplished by providing each with different Improvements, Wonders, Units and "Power Positions" that occur at different periods during the game.
If you have a game where the Human Player can play as other Factions in the game that are otherwise played by the AI, generally, the Human Player can play stronger than the AI. Because the Human Player can take advantage of things such as Lethal Land Bombardment (Lethal Shooting), Use Land Transports, Use Rebase for Land Units, etc... that the AI is not programed to use, this provides the Human Player with those different strengths. Therefore the AI should be compensated to contend with those strengths.
The problem is how to compensate the AI when a Human Player can play as the other Factions that are otherwise played by the AI. If we add compensation for the AI and then the Human Player plays that Faction, the Human Player has even more Strengths.
I think it is easier to set up basically equal but different strengths for each Faction that are played by the AI and the Human Player then have a Main Antagonist or Antagonists AI Player or Players that is or are compensated with more extreme Strengths to be able to contend with the Human Player. This is not only more powerful Units but more of them as well as providing more moves and or long airdrop distances for the AI. No matter what Faction The Human Player plays, the Human Player will have to help the other AI Factions of they will fall to the Main AI Antagonist or Antagonists. The AI will play like the AI is programed to play no matter what we provide it, so the key is What we provide it. So when compensating the AI Antagonist, we must ask ourselves 'What am I compensating for"? If the Human Player has Lethal Land Bombardment (Lethal Land Shooting) with Units and can sit back, Shoot and Kill AI Units without being Hurt then the AI needs compensation such as Many more Units and many that are far more powerful when directly fighting. Example: If the Human Player has 20 Units that can lethal Land Shoot and they have Blitz with 3 Moves each that would be a total of 60 Shots. Now what if the AI Antagonist sends 100+ more Powerful Units to attack? ...at least 40 Powerful AI Units would take a toll on the 20 Units played by the Human Player.
One of the most difficult areas to adjust is when adjusting and compensating the AI Civs that can also be played by the Human Player. This always requires a Great deal of game testing and adjustments.
There are many ways to obtain Variety between the Tribes, Civs, or Factions so they will have a different feel while being basically equal in strengths but Main Antagonists are easier to adjust and compensate if only playable by the AI.
In EFZI2, The Zombies are the Main Antagonist and although each other Faction can be played by the AI or Human Player, each have differences. As a Human Player, I can beat All AI Human Factions when playing as the Zombies within 32 turns to win the game. This is because I can take advantage of what the AI cannot. The Zombies are not meant to be played by Human Players but I allowed it for the sake of better understanding the AI Zombies and novelty.
Some of the Most Powerful abilities we can provide the AI is the ability to have more Units, the direct power of those Units and the ability to reach the Human Player's "Cities" and Units faster.
Rather than "beating your head against a wall" trying to have the AI build the Units you want them to build at any given time, take more control of the AI with Wonders and Improvements that Auto-produce Units when and how many you want the AI to have. By doing this, the AI is free to directly build the other Units they can. This creates more variety rather than the AI ONLY building Hordes of the same Units and not building others we want them to build as well.
The differences and "Feel" to each Tribe, Civ or Faction comes not only from Graphic differences in Units, there are Unit sounds and Music as well and although these differences are only sensory differences, they do change the "Feel". The direct strengths that we provide the AI is what really makes each Tribe, Civ or Faction more unique in Game Play, regardless of the AI programing. "Thinking Out of the Box".
...The thing about "Game Balance" is, there Must be imbalances within the game that varies between Civs throughout the game or there will be No Balance... Only stalemate.