The Freeborn are a mix of hi-tech C3, mid-tech AI and feral fighters from less-advanced worlds they trade with. Whilst not as directly tough as the AI nor as long-range a force as the C3, the Freeborn have access to an incredibly wide range of weaponry and upgrades that means each unit can be different. They have access to some of the C3’s drones and have very good access to support units, but can also make good use of a range of unusual creatures as well as solid, basic, core infantry and terrifying ferals. The flexibility of their command units is second to none.
From a background perspective, the Freeborn are the most flexible and (possibly) interesting as a player can create their own house, with its mores and customs. They certainly suit players who like extrovert forces or fluff, incredible flexibility and tailorability, but also those who like a variety of aliens and a flamboyant visual appeal.