First of all the group of Christians which don't believe in the divinity of Jesus are not as small as you think. Many don't even call themselves "unitarian" but when you ask "Do you believe Jesus is God" its not always a yes. Everyone interprets it how they want due to the various contradictory verses in the Bible.
Secondly the group would also have been much larger had the trinitarians not systematically killed them off during the first several hundred of years after Jesus' death.
I've posted several times that the differences in Islamic sects are in no way similar to the different interpretations of Christiany. Our differences are in certain rulings but there is no confusion over something so fundamental as the monotheistic nature itself.
The Sufis are a definite sect of Islam that believes that God is in all things and all things are in Him.....a mystical sect, but a definite sect nonetheless. In addition, they claim all visible and invisible beings are are emanation from God, and are not readily distinct from Him. We refer to this theology as pantheism and it's a drastic depature from orthodox Islam.
Sufis also hold the belief that man can attain divinehood by being absorbed into the being of God.
Do the Sufis represent the monotheist majority of Sunnis and Shi'ites? Certainly not.....in fact the only relationship to Islam is the fact the Sufis themselves just call themselves Muslims.....but, not really Muslims.
But where have we heard this kinda thing before? That's right, the Islamic position of Unitarian Christians representing the whole of orthodox Christianity....as mentioned previously you can also include Buddhist Christians, Islamic Christians, Naturalistic Christians, etc....again, not Christians.
So, as they say, it goes both ways.
I didn't even mention the Wahhabis, the Druze, the Alawite, the Ahmadiyas, Sikhism, Baha'i or black Muslims. Sure monotheism is a core tenet, but again they each hold to heretical and/or extremist views that do not represent the core of Islam yet they all are nominal Muslims. Now I'm not as arrogant as to claim to know the inner workings of these small sects, but they do exist and they do muddy the waters for Islam like many nominal Christians sects do for true Christians. That said, it really isn't appropriate to use these types of arguments because it does go both ways. I've left it alone until now because I keep getting thread after thread and post after post about Unitarian, Unitarian, Unitarian, Unitarian....ad nauseum.