I can finally help answer this!
1. Custom loops are way quieter. The D5 pump is AMAZING.
2. Having ran two AIO's in a system for a year, i can say a custom loop runs WAY cooler, WAY quieter, and fits in a much smaller case.
3. GPU support. With the exception of pure GPU cooling on the NZXT G12 (with no modern GPU support, and no active VRA\ VRM cooling), why aircool your 65W CPU when theres a 300W-400W GPU there? I cant get an AIO to slap onto my 3090's VRMs
4. The sheer customizability. Holy crap i blew a lot of money whenever i changed hardware and the goddamn AIOs would no longer fit, or the hoses were the wrong lengths. Oops sorry, hits your RAM now. New mobo? woops hits the oversized VRM heatsink, gotta go front mounted now. Gotta flip the AIO over to make the hose reach, oops now the temps are 15C higher (yeah, that all happened)
downside: i'm still crying at what i've paid, i resorted to GPU mining on the 3090 to pay for its water cooling upgrade
attached is the pics as i went through all this bullcrap with dual AIOs, , a GPU with an AIO that just goddamn sucked (dual AIO just sucks unless the hose length is perfect) then a CPU AIO starving the air cooled GPU with its stupid custom heatsinks to cool the VRMs, and finally giving in to custom water on the GPU - then realising the 280mm rad i had on the custom loop cant be top mounted because it hits the RAM, and having to get a slim 360mm for the top and a CPU block -.-
I have ADHD (and probably ASD) and random noises like rattling fans drive. me. f*cking. bonkers.
GTX 1080 with a kraken G12 and dual corsair AIO (cool, but noisy AIO pump. could not be oriented or located ANY other way, jammed into the 24 pin power cable and RAM)
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AIO GPU - crap hose length, frustrating fan cables, and ofc it leaked.
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Admittedly the native AIO GPU and CPU AIO looked SWEEEEET.
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3080 leaked, 3090 was the only thing in stock. Air + AIO? yeah 105C VRAM temps said oof.
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Lets fight with cooling this bitch when the AIO blocks the cool air intake...
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Custom water (mostly second hand parts) for GPU, GPU temps 40C lower. CPU temps... did not like the preheated air.
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What i should have caved in and done all along.