PanO: higher BS, lower willpower than usual, wide choice of TAGs, straightforward tactics
YJ:close-quater army, with higher CC and lots of short-ranged weaponry, best choice of power armors in the game, some very nice infiltrators too.
Ariadna: very low tech, but plenty of AP weapons, some cheap and fast troops and a lot of camouflaged models to choose from.
Haqqislam: great LI choices, very decent medium infantry, very good doctors, quite a few units with regeneration, a bit lacking in heavy weapons. Higher than usual willpower.
Nomads: dirty, sneaky fuckers with lots of wierd toys to annoy enemies. very good choice of medium infantry and infiltrators, very nice air-drop troops, a bit lacking in armor departament.
Combined Army: evil aliens, highly specialized, acces to a few more advanced weapons than humans, high point costs.
ALEPH: high point costs, durable and fast, but lack heavy or long-ranged weapons at all, very suseptible to hacking.