Attila Total War seems to have great gameplay, great immersion and is often very good looking. I'd say one of the best Tw3 engine games. Certainly better then Shogun and Rome II.
Yet still compared to these games it lacks something, accuracy. (not sure how accurate Shogun II is).
From Empire onwards Total War games have in vanilla had a greater emphasis on history than previous games. Culminating in Rome II where CA depicted more unique cultures than in any previous game (and fairly accurate most of the time).
The factions were almost all based on reality, of the playable factions only the Suebi and Iceni may not have existed at the start of the game.
And then we look at Attila, when it comes to factions it is of course better than Rome I but still not as accurate as the games mentioned above. The Ostrogoths for example certainly didn't exist in 395 yet they are playable, the Sassanid Empire is a lot of states instead of one (More understandable for the Seleucids as they had several independently ruling satraps), three minor Norse Peoples are regional powers in North-West Europe, a few of the steppe factions are innacurate I hear
and a few of the Roman succesor states are very implausible (nice concept I say but really Italia?). I think they represented the eras factions ok when it comes to whom they are.
However the real innaccuracy is in how factions are depicted, not what factions are depicted. I hear there are quite a lot of visual inaccuracies, some pieces of shields and helmets never being used and also a tiny variety in helmets.
The rosters are way weirder, making factions unique at the price of going beyond historical plausability. A few examples of anachronistic or non existing units are: Germanic Phalangites, All the "dessert units", middle age era east Roman
units (Tagmata, Menualatoi and I hear Varangians), generic Limitanei represented only as a low tier spear unit. And more..
Also many factions have no unique representation, for example the Brittons , the Nomads (as they are depicted, generic asian nomads), I hear that the Africans and Arabians share a roster of "desert units" but I'm not entirely sure.
As we see above a massive downfall since Rome II. A few possible reasons:
The low variety in unique cultures and models is almost certainly becuase of lack of time to work on Attila. Most likely they want as many as possible on the Warhammer game as soon as possible.
Some of the inacuracies are intentional to benefit gameplay: For example partly the differences betwen Germanic and Roman factions and the divided Sassanid empire. Since they want large rosters they have to look beyond realism.
A few of the inaccuracies are probably included to make the game appeal to more people, for example the Varangians are their partly becuase they are well known. And the Vikings becuase Vikings are "Cool" and you get many Scandinavian buyers (From my observetions many of my fellow Scandinavians dont care).
The question now remains, is it more or less accurate then lets say Barbarian invasion. Wich also had inaccurate rosters and factions.