Regarding Ming units I've been reading the blog Great Ming Military, which explains the weapons and armour used by Ming armies. It has an interesting section about what is and isn't historically accurate.
http://greatmingmilitary.blogspot.co...h-post-p1.html
http://greatmingmilitary.blogspot.co...t-post-p2.html
I plan to add some textures from Song units as I have a Song dynasty Mount and Blade mod, and the Tusi and South Ming soldiers did wear some older style armour during this time period.
Regarding ships in addition to the problems that have been mentioned each ship is split into 4 parts (bow, stern, starboard, and port) and each part has multiple versions to reflect different levels of damage. So for each new ship I'd need to make 3-4 versions of it. This is why so few new ships have been added.
Regarding Korean units most of the high quality Ming body parts and textures are from a Mount and blade mod; I didn't make these units. For Korea their units are either recoloured Ming units or are made using modified Shogun 2 body parts. As a result it's not easy for me to replace the existing Korean models and texture with higher quality models and textures because I'm not able to make them.