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Unfortunately, your question has no answer. Both have errors. Both can be unreliable. Your choice really depends on how you plan on using them.
Satellites instrument contain sources of error, such as bad observations (wildfires in the NIR), mapping problems (especially near the poles), and representation error. Don't underestimate those sources of error- ...
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There are numerous short modules offered by UCAR's COMET/METED website: https://www.meted.ucar.edu/training_detail.php
Modules specific to GOES-R include (but aren't limited to):
GOES-R/JPSS test cases for convection initiation and wildfires.
Advanced Baseline imager
A satellite foundational course
There are 19 courses with GOES-R in the title.
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No, cloud fraction is not a good measure of $\ce{SO_2}$. Clouds are functions of moisture, temperature, and cloud condensation nuclei. Som of the other variables, such as (but not limited to) ColumnAmountSO2 or ColumnAmountSO2_TRLare better indicators.
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