Since Edi's asking, here are a few things I think could be improved with Machaka:
One think I wish Machaka would do would be to more fully embrace Ashanti mythology. The connection between an African Nation utilizing spiders, and Anansi the spider-god (who originated in Ashanti myth), is pretty obvious on the face of things, but it's also portrayed rather vaguely and tentatively.
They use/are related to spiders, they're African.
A third step into "they're the Ashanti myth of Anansi, through the lens of a historically-based fantasy strategy game" is never really taken, and I think that is part of the reason they come off as a bit of a steriotype. It wouldn't be impossible to change just a very few details, and end up with a South American Nation riding Goliath bird-eating spiders, or Australian Aborigines riding funnelweb spiders, or Laotian Giant huntsman spiders, particularly since tarantulas are
American in origin (the word "tarantula" was first coined by American colonists for the funnel web tarantula, and tarantulas range from the Caribbean to South America.
If Machaka were Caribbean in origin, they would make a lot more sense, thematically, since the myth of Anansi was carried into the Caribbean by African slaves. Caribbean flavours could then be added to the Nation, giving it a stronger and more defined "theme".
Another problem is that you've got a Middle Ages Nation using what amounts to an Early Ages sacred unit (big, powerful, expensive), without a very good reason for them to be incorporated into that Nation's armed forces, and without corresponding Middle Ages technology (no crossbows, little armour).
If they're going to remain Ashante, Machaka might be given an Asantehene "king" unit, who would wear a soulwasher's badge, which would give them strong protection against demons and the undead, up until they were first struck in battle (1 HP Firstform, second form lacks the soulwasher's badge).
Okomfo Anokye was a powerful priest and co-founder of the Ashanti Nation, who occupied the positions of both Nation Founder, and Lawgiver, and who occupies a "Merlin-like" position in Ashanti legend:
http://en.wikipedia.org/wiki/Okomfo_Anokye
Apparently (from what I could tell from the article), he had mystical power over the earth.
Having a 3 Holy/3 Earth hero would give Machaka a boost.
The other co-founder of Ashanti was Osei Tutu, who was a great warrior, and who possessed mystical amulets and body armour that were proof against missles--when he left them at home on a raid, this proved his undoing. So, you might have a strong (human) warrior who comes pre-equipped with an amulet that protects 100% against missles.
A nice heroic unit to give them from Ashanti legend would be the sacred Golden Stool, a unit that might give them a strong banner, a patrol bonus, holy magic, blood magic, etc. Whatever seems to best fill any percieved gaps. The Golden Stool is such a significant and sacred cultural item that the Ashanti have been known to go to war just to protect it's sanctity.
http://en.wikipedia.org/wiki/Ashanti#Ashanti_Kingdom
Another heroic unit to give them could be the two golden masks of Adinkra, which were made from a golden stool that Adinkra, the king of Gyaaman, had made to replicate the Ashanti stool. The Ashanti defeated Gyaaman, beheaded Adinkra, and then made two masks in the shape of Adinkra's "ugly face". They now hang on either side of the Ashanti stool, to this day. They might come with banners and high Fear, and be intended to "lead the way" at or close to the front lines.