There are numerous denominations within Christianity with slightly different “takes” on faith matters. But as long as certain non-negotiable ideas are included in each denomination’s program, they get along fine with all other Christian denominations.
In Islam, it’s different. Sometimes, there is all-out war between Islamic sects, and ugly accusations that other sects have things all wrong and their adherents won’t go to heaven.
Here’s a good article explaining the basis for the many sects within Islam and why they clash:
Differences between Sunni and Shia Muslims:
Salafism vs. Sufism:
Reports: Ultra-conservative Wahhabi Muslim ideology (Salafism) is being spread in American mosques through an estimated $100 billion investment by the Saudi monarchy:
The Wahhabi sect of Islam is said to be the source of an estimated 80% of the radicalization in American mosques:
Saudi money goes to build mega-mosques in the U.S., and then, following the adage “He who pays the piper calls the tune,” it is radical jihadi-style Wahhabi content that is preached and taught inside those mosques: