The main problem with huge air coolers is physical clearance with the top GPU card. RAM compatibility isn't such an issue because 99% of us will actually do enough research to ensure proper fit.
Most GPU coolers require good airflow around and above the card. Most are designed with the "hidden" requirement that a 120mm or 140mm rear exhaust fan sucks air right off the GPU backplate. A large CPU cooler like the NH-D15 will blast a lot of air through a CPU rad but it will also block air needed to properly aircool the adjacent GPU slot.
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