Only 2/3 of the gpus are still hybrid. The others are full waterblocked. Yes those hosts are bit quieter but you still cannot call them quiet with every fan at full speed on the radiators.
And it is a lot easier to get hybrid cards than full waterblocked cards. Plus you incur an extra expense starting with an air cooled card and then converting it. Cheaper to just get the full waterblock version of the card in the first place.

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Water cooling is still much quieter and more efficient in keeping temps in check for 24/7 100% utilization BOINC computing.
You'd have to have 4000 cfm to keep all the gpus and cpu's cool enough to not throttle with air cooling.
The gpus and cpu clock much higher with the lower temps of water cooling.
I'm using 1250W with just the 3 gpus and the cpu on of the big iron hosts and generate about 6000W of waste heat from the farm. No need for central heating in the winter, the farm is more than sufficient.

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That is impossible to achieve on a multi-gpu host. Standard 2 slot cards leave 1/8" of clearance to the next card . . . if that.
You've obviously never run a multi-gpu host populating every possible board slot.
Air cooling is not sufficient unless you are happy with severe downclocking because of high temps.

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I would never use risers. Only natively in proper motherboard slot positions. I have a teammate that is constantly fighting his attempts to use risers. Always flaky and never stable.
Install cards in their proper slots and set and forget. Never an issue and cards run at least at X4, and mostly at X8 and X16 depending on the motherboard and cpu.

Also I mostly use workstation motherboards and cpus. I have run 4 card hosts for a very long while now.
My other teammate has run 6,7 and 10 gpu hosts in the past in mining chassis with full size X16 risers with good results.

I just use large PC cases with good ventilation and many radiator position options.

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USB cables are the weakest links in using risers apparently from the issues reported by my teammates. Next comes PCIE cables and splitting the power loads up among several power supplies.
Don't skimp on the USB cables, buy the best, shielded preferably.

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All was meant if powering several gpus, don't try and throw them all on a single power supply. Get another power supply and gang start them together and split the loads between the two power supplies.

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The main problem with waterblocks is still insufficient cooling.
Most waterblocks only cool the top part of the GPU.
The bottom part also needs cooling.
They also sometimes only cool the GPU core itself, not the RAM.
The RAM needs at least a 80mm 12V fan running at 5V - kinda stream of air, over a heat sink solution.

It's always best to use a hybrid system.
Curve bladed 12V fans on 5V (usually case fan speed set to ~20%) make less noise than the water pump of a water cooled system.
They can also improve the cooling of passive cooled water systems, or even passive cooled GPUs, like GT 1030s or maybe even more future GPUs.
They use less than 0.5W to run, so it's a no-brainer to use.
At 12V they may use up to 2.5W, which is about 1% of a modern GPU's power draw.

If a custom water block for a gpu doesn't cool the VRM and memory as well as the gpu core, it is a very poorly designed waterblock. No well known marque brand waterblock has those deficiencies.

Most waterblocks either can use the stock backplate from the air cooler or even have actively water cooled backplates for the cards like the original RTX 3090 or the Kingpin 3090 with RAM on the back of the PCB and has very high temperatures even on the stock air cooled solutions.

https://cdn.shopify.com/s/files/1/0032/5530/4241/products/optimus-kingpin-gpu-block-backplate-2_2000x.jpg?v=1627528303
https://cdn.shopify.com/s/files/1/0032/5530/4241/products/optimus-kingpin-gpu-block-front-2_2000x.jpg?v=1627528303

Or have custom backplates with proper cooling.

https://cdn.shopify.com/s/files/1/0032/5530/4241/products/optimus-gpu-waterblock-ftw3-xl-backplate-1_2000x.png?v=1628103290

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I remember that I used to buy special cooling kit for my work station when I was gaming addict, this picture reminded my old days.
Old is gold, it is right.

You don't have to be a gaming addict to want to water cool components.

Cooler temps = higher clocks plus more power headroom.

I don't game on my PC's but I do water cool every one of them for BOINC crunching.

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