Almost everything faced in the game either is derived from human tech or simply is human, and of the others (the dropship and strider) one's just a light transport (which is able to take multiple anti-tank missile hits), and the other's simply programmed in game to not actually hit the player, lowering the perceived threat quite a bit. We do see a couple of clearly-combine units within the citadel that look like some kind of armoured ground combat... things, but they never appear at any point afterwards and aren't seen actually doing anything other than moving on a conveyor belt.