My last game as the Roman Empire, the Iberian kingdoms only fell in the 1190's as western Europe fell into catholic-orthodox civil wars (which soaked up all the holy orders). Before that they were doing fine, thanks to French help and holy orders.
I've only witnessed France falling to Moslems once and that was a pre-1.07 game.Yes,Christian in Iberia are weak.Yes,France can have some vassal problems.Even starting in 1066,I generally see France Surviving.Iberian Christians Getting Steamrolled and then France or HRE Steamrolling the Moslems.The GERMAN Kingdoms of Mauretainia and Mali are more of a problem.Factions have slowed the blobs a bit but I think that Moslems might be UNDERPOWERED! Their Blob the Fatimids now regularly get ROFL stomped by the mongols and Gets ganged up on by the Christian blobs.
In most of my games the mediteranean becomes a Christian lake.By 1300 the moslems are like Pagans at start in 1066.