Ok guys, I qualified my post after carefully watching the videos, especially the Norman campaign one: the devs actually did some research but almost only for the Late Middle Ages. Aside the nice late medieval knight spotted by @GepardenKalle, there are three other remarkable European units, late medieval too:
On the left we can see knights on foot armed in what look like mid to late 15th century Italian armors (which were imported all across Europe) ; in the middle late 14th to early 15th century European knights with their typical bascinets ; on the right some infantry unit in mid to late 15th century armors again.
Also, I spotted an Indian helmet identical to a model widespread in 15th century Iranian artwork:
All these units/elements look very good, and I wish they put the same effort in most units.
Of course, such late medieval designs aren’t suited at all to the Norman era, but this is due to the game mechanics (the game reaches its max potential in Imperial Age which is based upon Late Middle Ages/Renaissance - a campaign struck in the Feudal Age would be boring gameplay-wise).
Edit: late medieval European ranged units aren’t bad either:
Unfortunately the rest is very poor except for one or two elements.
Let’s start with the Dark Age Scout, who, on contrary to what suggests the pic found by @CaptainMiguel isn’t accurate nor does it look like my sketch apart from the cap (that I also chose so as not to diverge too much from the devs’ design).
His equipment consists in a short tunic with a sleeveless surcoat put over, tight chausses with rather high boots, chaperon, cap, broad type XIV short sword, stirrups and a very modern-looking saddle. He also have long hair and a beardless face.
Stirrups can work as they were seemingly introduced in Europe around the 8th century (I didn’t include them in my design so as to highlight the low technology of the game’s Dark Age). Same for the cap which isn’t shocking.
But none of the other elements suit the European Dark Ages and many are mere clichés.
This unit could have made a good Imperial Age Scout though.
Dark Age Infantrymen now.
Basically the same outfit as the scout. Heater shield, type XII or XIV sword, tight chausses with shoes. Again, this has nothing to do with Dark Ages. Although quite inaccurate, it would suit Castle Age well with that High Middle Ages vibes.
It’s a pity because there are lots of cool designs possible for Anglo-Saxons.
Feudal Age cavalry (I presume):
This is really a wasted unit. Its elements are mostly accurate: short sleeved hauberk, very nice kite shield, simple spear and possible legwraps. The leather hood is probably inaccurate but still fits the period well.
Unfortunately, and probably for readability’s sake, they added that sort of colored half surcoat which, combined with the lack of conical helmet, totally ruins the “Hastings” vibes of the unit. A partly colored helmet like on one of my alternative sketches would have been a much better choice really.
Also, this unit attacks with the couched lance technique, which really become the norm a bit later. The Bayeux Tapestry often shows the spears brandished above shoulder - I would have kept this technique for the Feudal Age and left the couched lance for the Castle and Imperial ages to highlight technological evolution again.
William the Conqueror?
Visible in the Norman campaign video. It has nothing to do with the 11th century, fully late medieval look again.
Some clichés units:
While they aren’t shocking, there is little to no research behind. I like some of these designs though.
Castle Age units:
Not bad at all, but again… late medieval with such plate armors. Such designs should be limited to Imperial Age only. The sword is quite high medieval looking and as such suited to the Castle Age but I hate those long grips the knights’ have (and I doubt it’s an attempt to portray b.astard swords).
Indian infantry:
With their Kulah Khud helmets, shields and modern swords, they look straight out of the 18th-19th century.. Also I hate how thick the shield model is.
Finally I can’t really tell for Chinese and Mongols, not my field of expertise, but some forum users said they too got mostly 18th-19th century equipments.
So I would reformulate my criticism: early and high medieval era armors/equipments need some love too, all the more as they make up 3/4 of the game’s timeframe
Relevant note. However these anachronisms are a bit too extreme and I therefore doubt this mechanism explains them.
Another possibility is that the evolution in the game is actually merely technological despite the historical names of the game’s ages.