I purposefully had them as an AI in my current game and they didn't fare too well. They went authority and were out-founded by myself and a couple other non-religious civs. The only plus for them was Inuit (who've performed horribly 2 games in a row now for AI...) being sandwiched alongside Songhai, so they cooperated for the first half of the game to wipe him out. Askia reaped almost all the benefit, and it was a short while before his religious pressure and missionary spread eroded any religious gains from David. He ended up voluntarily vassalizing to Askia, and now I've got a monster Songhai to repel for the remaining 100 turns. I used Israel months ago, and won a CV game on King, but there's been so many tweaks over that time so I'd have to play them again before coming to any conclusions myself.