I have no idea what people are complaining about.
Given i have only played 2 serious run's , One as france and the other as britanny.
Either way in both games the Muslims are a joke. As France i completely crushed them. Utterly destroyed the African muslim nations and was easily able to take and hold Egypt + Jerusalem. The only threat on the map , as it is in most games is ERE , and a various times the Khans.
As Britanny the Muslims are getting owned by Iberia + France + Me (conquered England and Ireland with ease , now just rofl stomping the east pagans).
Overall i think the Muslims are still far too weak. They just don't seem very strong considering they are supposed to be really advanced in their tech's. My one complaint and thing i agree with is i think ERE and HRE pretty much never crumble. They never pose a direct threat to me in any of my games , and its usually easy to get a claim on HRE (they marry their princesses off like they are nothing) and wait till a civil war to take it (which makes the game boring once you have it). But if the player doesn't intervene , none of the AI nations like Poland or Hungary or even the Muslims ever take advantage of HRE and ERE's periods of immense weakness. So i don't think the problem is with them being too strong or vassals not rebelling enough (they rebel every time emperor changes). I think the issue is the fact the AI are content to sit within their own little kingdoms and the Muslims are only ever interested in Iberia , and any occupied lands that are de jure part of the muslim world (successful crusades). Maybe the Muslims were not historically invaders (i really don't know) , but it wouldn't hurt to see other countries get a bit ambitious.