If you're using iaijutsu focus you will probably be using a quick gnome razor, since that allows more frequent uses of iaijutsu damage (but does not do that much damage, making it bad for a blitzer). A lot of factotums who do not want to spend a feat slot will stead choose a feycraft fineasable weapon. I have never seen a factotum wield a greatsword, so they might not wield one.
So have I which is why I included it in every dice calculation. Not all DM's do however.
http://www.giantitp.com/forums/showthread.php?t=113400
Average rolls do not allow you to win initiative, hit and get 3d6 on iaijutsu focus in the same round.
Average rolls does not mean you roll a 10 on everything.
Everything about the build involves dealing a lot of damage in the first round. Not every factotum takes knowledge devotion, some pump FoI to be able to endurantly use IP throughout the battle.
Not every factotum wield a great sword.
Not every factotum is allowed to take iaijutsu focus and use cunning strike multiple times.
I already explained why.
Weapon finesse + feycraft? Or thats what they want their character to wield?
Many DMs feel the "jack of all master of none" implies a factotum should never be able to out SA a rogue.