I think West Africa has the potential be really powerful based on retinues... Their cultural retinue options are, I'm pretty sure, the same 600 retinue-bux that a skirmish retinue costs. That means your retinue be very large (and honestly a 20% size difference seems to trump almost any other factor, and is particularly important for chain-sieging) and still benefit from the cultural building upgrades.
I don't know if this is actually true, since my west african game ended in a succession quagmire... but since my purely skirmish retinues are rocking even in HI-centric Scandinavia, I'm pretty sure cheap cultural retinues are good. EXCEPT, last I read, the west african religion gets neither the offensive nor defensive religion buffs. An acceptable loss, I think... The bigger issue is probably how undeveloped Africa starts out settlement-wise.
Something weird happened in my current game: I got the loyal eunuch servant event, which was good timing because I had just kicked the teutonic knights out of Sjaelland and ~15 holdings were burning a hole in my vassal relations.
(on that note, some jerk catholic king gave *three* counties to the Teutons over about 5 years. De-jure Dutch lands, still Norse, right next to my recently formed and reformed Norse Scandinavian Empire. He only actually has 2 counties left, he gave over half of his lands away... I guess he knew I'd take them anyway)
The weird part is that I gave the eunuch the Duchy of Sjaelland with "Include Lower Titles", but he only received the Duchy and the holdings... I still have all of the counties in between! Also he had no lands before, yet he formed a *merchant republic*. Probably related. I'm not sure it's a bad thing, but it does mean I have two trade republics in my empire now. The other one, located on the south of Finland, hasn't made any trade posts yet despite me investing two levels into trade practices. I think the technology hasn't spread to them (despite it being a few decades).
I'm just going to grant the counties and enjoy having a new trade republic with permanent +100 loyalty (at least until his house dies off because HE'S A EUNUCH)
Edit: Oh... I can't grant counties to eunuchs because they can't produce heirs. *Also* I can't have any more counties in vassal republics (max is 10%). Well... I guess it's okay though? He chose one of Sjaelland County's three cities as a capital, and I guess he's totally running a merchant republic. I'll see if he actually manages to establish trade posts.