According to this site, nubians and berbers aren't equal when it comes to weather :

Nubians are 1 Scrub/1 Sand/0 Forest/-1 Snow,
while
Berbers are 1 Scrub/2 Sand/-2 Forest/-2 Snow

Meaning that while Berbers are better because of hardy and schiltrom, once you move up north and leave the Sahara, Nubians seem to have their point as well.

As for Druzhina and Boyars... flavor ? The Rus roster isn't that large to begin with so...