Hopefully each faction is actually different enough from each other in a way that matters. It better not be a quantity over quality thing.
Bear in mind that there will only be, at most, eight playable factions. Remember all the minor clans in Shogun 2 that weren't playable? I'm guessing its going to be like that. In Rome for example, the Macedonians face off against a few rebel towns and the entirety of the 'Greek City States' for control of Greece. In Rome 2, I imagine Macedon, Thebes, Athens, Sparta, Epirus, Syracuse and all the others will be their own factions, rather than get lumped together as 'The Greek City States'.
The +100 faction figure also likely includes emergent factions that appear through revolt.
Edit: I really like the idea of having lots of minor tribes and kingdoms, it makes power struggles far more interesting. Its also fairly amusing to see a once tiny tribe rise up and compete with on of the bigger factions. In my current Shogun 2 campaign, both the Shimazu and Otomo have been wiped out by a minor clan called the Ito.