In CiV there are a number of different factions you can choose from when you begin your game like Greek, Roman, Ottoman, French, etc. Each of these factions has some special bonus to make their style unique and 1 or 2 special units that are upgrades over the normal ones. Depending on what sort of terrain you start in, what sort of game style you are playing, what happens during the game and who you are facing each of the different special bonuses can be very useful, though certainly I have opinions on which ones are the most powerful! (Hint: Americans are bad!) A really tricky part of the evaluation of how good a faction is must include when their unit appears in the game timeline. For example, the Greeks get Companion Cavalry (CC) which are a mounted unit that appears early in the game. You should get access to CC in nearly every game you play and they are really powerful so having them is a big advantage. On the other side of the equation is the B-17, which is an American special unit that appears when you are able to build bombers in the modern era. As such by the time the B-17 is available usually one side has already won the game or is in such a dominant position that nothing can change the outcome. In many cases you will wipe your opponents out or be wiped out before you even get access to muskets so the B-17 is almost always going to be irrelevant.
The trouble with this sort of situation is it is pretty much impossible to balance it. I am trying to imagine how good a B-17 would have to be to overall be as useful as CC on average over many games and the idea is hilarious. Imagine if the B-17 were to be improved so that it instantly destroys anything it attacks and is completely invincible. This would mean that generally speaking you still never build one and when you do it is just to have fun bombing your opponent's musketeers into oblivion. In 90% of games it would make no difference that the B-17 is totally unstoppable. However, in the last 10% of games where you get to use modern weapons and the game is still close it would be absurd - the Americans would build B-17s and automatically win from that point. Having a special unit turn 10% of games from 'close' into 'rout' isn't even good enough, and yet clearly if I was in a close game and then my opponent built an invincible unit that just beats me I would find that terrible and ridiculous. It turns out that it isn't possible to make the B-17 powerful enough to make it a strong faction bonus without making the game completely stupid once it does get built. Of course these same objections exist with the Japanese Zero jet fighter, the German Panzer tank and a few others.
So in theory what you do if you want to assign a unit like a B-17 to a faction is you just make it a little bit better than the other units available to everyone else and then give that faction another really powerful unit or an amazing special bonus; you find some way to compensate them for the fact that their unit rarely matters. What actually happens in the game though is some factions (Greeks!!!) have early game units that are very powerful and so they are just much better than the factions that have late game units. The faction balance in CiV isn't wretched by any means - there are lots of interesting choices and even now with a lot of games under my belt I find it challenging to figure out the best faction for a specific strategy. Unfortunately there are a few lame duck factions that although they would be fun to try out to test they aren't really going to be competitive.