The "unlock factions by completing campaign" is long gone. It was in rome and medieval 2, but could be circumvented by 20 seconds of file editing. And in empire you didnt really unlocked the usa as faction, cause it was a different campaign.
But completing that campaign did unlock the USA in the Grand Campaign.
Yes, you can enter the code in order to unlock other factions, but those are mostly not designed to be playable - so there aren't campaigns around them, they aren't necessarily balanced to play in single player (think Spain in Rome TW), and they don't have the same variety of options. You can unlock the minor clans in Shogun 2, but you get a whole lot of identikit clans without special rules or unique units if you do. This is likely to be a bigger issue in Rome 2 than in previous releases since the playable factions are so distinct from one another, not only in units but in faction special rules. Unlockable factions are designed from the start to be playable.
As for going into the code to unlock factions, you can if you like, but why would that act as a barrier to the developers including unlockable factions? It probably doesn't take them much longer than that to create the code that locks specific factions in the first place.
EDIT: With news that R2 will have 117 factions, there's more than enough room for 8 starting factions, a whole bunch of unlockable factions, and as many DLC factions as they want even before they get onto expansion content (here's hoping for Britannia Total War as an expansion campaign, focusing on the aftermath of the Roman withdrawal in a similar manner to the early stages of the board game of the same name).
its dlc now. in empire with the warpath campaign, the peninsular campaign for napoleon, and 3 clan sfor shogun 2, 4 for FotS. So im pretty sure that 9nd faction for rome 2 will be a pre-order one, as the hattori were for S2
The Warpath, Peninsular and Rise of the Samurai campaigns are separate campaigns - and except for Rise of the Samurai use different maps. That's a wholly separate thing from unlockable factions.
Agreed that, given a set roster of factions, having lots of unlockable factions limits what they can add (and charge for) as DLC, but there's plenty of scope for having both unlockable factions and DLC factions, particularly since it looks as though Rome 2 will have more factions overall than Rome (there are no conglomerate Britannia, Germania, Gaul or Greek Cities factions, so each component group within the equivalent Rome faction is likely to be a separate faction in R2). Both Rome and Empire had a mix of playable, unlockable, and never-playable (without hacking the code) AI factions, so there's plenty of room to have - say - the Seleucids as an unlockable faction but Spain or Scythia as DLC factions.