The 'Balance' to really strong early units like the Gigir (or Zhou's Zanche, which is not bad either) is that they require 2 different Resources early in the game, when having both Horses and Copper available in your 'core' territories can be extremely problematic: I have managed it about 4 - 5 times out of several dozen starts, which is why I rarely ever pick Hittites, and in fighting an AI's Hittites I have never yet had to face Gigir.
On the other hand, the Hittite's +1 Combat Strength Legacy trait can be more than annoying when you are facing an entire army of units with it on good terrain, but without the (situational) Gigir they really have nothing else going for them except some fortified Outposts (Awari) which are pretty marginal if you don't have a unit or two around to support them.
You have to build their Emblematic Quartere, but the Olmecs with their Heads get two of the critical 'currencies' in the early game: Food and Influence. Harappans get more Food, but the Olmecs can expand faster, all else being equal, because they'll have the Influence to slap down Outposts faster, and attach them or Upgrade them to Cities faster. Since the Olmec Heads gives Influence for an adjacent Farmers Quarter, AND intrinsic Influence, and they get Influence per Territory, they can easily pull in twice the Influence/turn of any other Ancient Faction. That's not to be sneezed at.
In general I think Ranged Units (like the Olmec Javelin-Thrower) need a hard look, because right now the Line of Sight, Indirect Fire rules make little sense and result in some very peculiar 'Upgrades' for some of them. I know there has been some discussion of this over at the G2G forums, we'll see what comes out of it . . .