I think Kostic is working hard on western troops. Aside from that, "muslim" factions were and are always a problem in Medieval 2 and its mods, they are mostly the same and many things look copy-pasted. Its not only faction leaders that are wrong, but entire rosters are made up by looking into Osprey books, which are highly missleading. I don't think anyone could "fix" this problem, it just requires a lot of work and studying the source material.
One example from SSHIP. I am using 0.97 version.
These here are helmets for the general's bodyguard you start with. One of them on the right has a sliding nasal, which is started being used in 14th century, but really shines in 15th. Game starts in 12th century, sliding nasals have no place then.
Take something like this. It cannot exist before 14th century just because of the sliding nasal.
This is how helmets with face-covering aventail would have looked like before 14th century. No sliding nasal. This one goes way back to sassanid times.
These here are late 15th century turkmen on the left with sliding nasals. 14th century Il-Khanate with no nasal and with solid nasals in the right.
Now about the other helmet, shown on the left in my original image.
These are various illustrations from Osprey books, which i said are missleading.
This is a reproduction of original helmet, which is from national museum of Hungary. As you can see, it does not fully resemble those, which are shown in Osprey, but that is not the main issue. The problem is, its a late 13th, early 14th century helmet, left in Hungary after the second mongol invasion in 1280s, or brought in my maybe cuman, maybe tatar mercenaries. Its decorations may be indication of Il-Khanate origin. This helmet really has no place for any faction in 12th century, yet it is worn by general's bodyguards and various officers in units.
Here is another example from Olen Kolodets burial. 14th century golden horde helmet.
Here is also helmet from Ploskii. Also 14th century golden horde helmet.
P.S. This is related a bit to one topic.
This is mongol helmet found in Yesenovo. It either is missing a brim or had none. The problem is, many people are attributing this to byzantines. I know, right? Well, its not byzantine, its mongol and it has similar contruction like Ploskii helm. https://www.levantia.com.au/helms.html This article is what popularized the opinion that Yesenovo helm is byzantine. Itsmostly incorrect, do not use it as a source for anything.
In the end, i would say that factions like abbasids have more than their generals that are incorrect.
These are Abna Heavy Spearmen and they wear mail and plate armor right at the start of campaign. Mail and plate armor is from 15th century and onwards (although it can be rarely seen in late 14th century on some dubious findings). As you can see, fixing muslim rosters is hard and almost impossible task. Kostic is already working too hard.