In my campaign with this mod, Muslims became nr 1 power around turn 20 - 30. Sassanids got beaten quickly. Romans crumbled, lost Asia Minor to various rebels factions, possibly liberated by the Muslims.
In turn 50 Muslims had 47 settlements, and stopped growing, probably becouse they were surronded by military allies like Libyan Tribes. They did try to expand into the steppes, defeating Great Bulgaria on their way. My faction, Korotan, stopped their jihad in Dacia. After they lost like 5, 6 armies, they didn't try again. Perhaps without human player as an obstacle on their way, they could conquer more.
At turn 100 they hold like 46 settlements - all the way to the estern edge of the map. They waged a war against Goturks but didn't try to finish the Eastern Romans so far. Btw, great cmpaign and balance, played on hard and AI was active but not overpowered. Muslim garrisons were very tough, so i didn't really try to conquer them.

I never had such a Muslim expansion. The point is, that the rulers have random attributes. So any campaign is different. In other words, when a ruler changes, the settings of the new ruler are random. Very worse is defensive. Thanks for your report. If you want to test my new unpublished mod with many innovations, you (and others) are welcome on my Discord.
https://discord.gg/SBfYmpDD

islam is always desroyed by sassanid. i tryed 3 times.
i never combat this faction, even with this submod "islam stronger" (nothing difference than without)