Even though the "style" is different, it is still relatively accurate.
The Daimyos that get the unique Samurai T1s were all fairly significant Clans from the Sengoku Jidai, so that is why they are special.
The rest, though they don't get Samurai, are still accurate by representing the Ashigaru, who were the peasant soldiers on the Japanese military.
Pretty positive on all of this, but @Grand Historian would be in a better position to answer
Ashigaru began to supplant the Samurai in nearly every Sengoku army - the Tier 1 model for all the non-unique clans also being a samurai, just of the kind that's not often shown in popular media (the average samurai in the sengoku period, in fact, preferred polearms to swords and usually couldn't afford a complete set of armor) - so it's a pretty accurate representation of the evolution of the armies. All the selections for the sprites were simply picked for the most important clans of the period that start off with land (hence why the Oda and Tokugawa don't have anything) - the only clans I'd say who should have gotten unique models were the Amago and Satake over the Hatakeyama and Shiba; the former becoming major regional powers as well (the Amago even came pretty close to marching into Kyoto) while the latter two really did nothing but have civil wars and loose land after the Onin War (with the one exception of the Noto-Hatakeyama, who flourished for a few decades under Yoshitsuna).