To me, zombies in fantasy like this seem more of a supporting "Character" like how the dragons are (right now atleast). The zombies can be summoned with magic to help as a "mercenary" if you will. If they were made as a faction, something/someone non-zombie would have to control them. Like in "dragon age" lore the darkspawn are unified, but only because of the archdemon other wise they are disorganized, attacking everything including themselves.
"90% of what is considered "impossible" is, in fact, possible. The other 10% will become possible with the passage of time & technology." -Hideo Kojima