The problem with the Abbasids is that they're very large, but their ruling dynasty is ridiculously secure because of all the positive opinion bonuses they get (Sayyid, lots of Piety from being Caliph, lots of prestige from inheriting a gigantic empire) and their expansion is quick because of incredibly strong CBs. The only limitation is their military strength and decadence, but decadence vanishes when you're winning holy wars left and right.
The Muslims really aren't balanced at all, and never have been because of their weird mechanics. One patch you have the Fatimids marching through Constantinople, the next all of Africa and the Holy Land is a patchwork of HRE/Castille/Venice colonies. By TOG and CM, I've only seen them ineffectually collapse in on themselves and be pushed back hard (the earlier patches of TOG) or else occupy Turkey, Greece, the lower Balkans and half of France before 1000 (the current state of CM, last time I checked).
They aren't even that fun, because all their neat features carry with them the incredible bore of decadence micromanagement and having to murder all the relatives that slide back into decadence after being convinced to reform, as well as all the relatives that your idiot cousins tried and failed to reform. It's only worse with you basically requiring a full 4 wife harem and not having any Chaste trait, meaning that within a few generations you're combing a family tree of several hundred members for a handful of decadent members ruining things, until it gets to the point that while your vassals are completely loyal and your army can crush all infidels underfoot, you're basically spending 90% of the game dealing with relatives.