'mainly stem from' is correct and usable in written English.
It is often used to describe the cause of a particular circumstance or situation. For example, "Many of the environmental issues we face today mainly stem from human activities."
And my disappointments mainly stem from the systemic horrors of our political/governmental structures.
The increase will mainly stem from the recent rise in petroleum prices, as natural gas increases lag those for oil.
The commercial uses of alums mainly stem from the hydrolysis of the aluminum ions, which results in the precipitation of aluminum hydroxide.
They are written in one of the Semitic languages, Old Assyrian, and mainly stem from trading centres such as the ancient city of Nesha (also known as Kanesh; now Kültepe, Tur).
Discrepancies in results mainly stem from restrictions of experimental designs.
Deviations mainly stem from the Reynolds number dependency of the flow distribution.
The level playing field, such as it exists, has potholes, mainly stemming from conflicts of interest.
This resistance mainly stemmed from false rumors about the polio vaccine.
That mainly stems from current manager Arsene Wenger having another year after this season to run on his contract.
The problem in the rural areas mainly stemmed from the inability of the Red River to absorb overflow from smaller tributaries, which are backing up and spilling over.
The collapse of Mosaic into administration – which follows the demise of a number of UK retailers including Woolworths, MFI and Land of Leather because of unsustainable debts and poor trading – mainly stemmed from the financial and economic problems in Iceland.
