There is generally an air-cooling strategy with a heatpipe cooler, or a water-cooling strategy that ranges across AiO, AiO-customizable, and custom-water cooling kits.
There may be some water-cooling configurations that benefit from a pressurized case. Others choose not to concern themselves with it.
Air-cooling through heatpipes can benefit from pressurization.
Since Water-cooling radiators installed with case-intake fans provide a marginal advantage over (top-mounted, for instance) radiators and exhaust fans, water offers additional expansion which improves cooling to the degree it represents doubling radiator capacity.
One thing that COULD be missed by water systems is a cooling of onboard voltage-regulation and other heat-generating components. Motherboard and other components still need airflow, and to do it better would apply techniques I might employ in air-cooling -- with ducts, singular placement of a single intake fan, or anything else that at least complements airflow with radiators.
I block off vents that don't assist with pressurization. I overwhelm the case with intake CFMs (and some minimal but optimal number of intake fans. I augment my CPU cooler with an accordion duct and a 120mm exhaust fan from a class of fans that would make others uneasy because of "noise potential" -- a Nidec-Servo Gentle Typhoon AP30, for instance.
I believe my air-cooling and the advantage of a binned and CLU-relidded CPU rivals the performance of $200 or even $300 water-cooling systems with retail-stock CPUs, until you add more radiators.
In my Stacker 830, I have a motherboard duct exhausted from the right side-panel with the Stacker's Cross-flow accessory barrel fan. I have four 140mm decent quality intake fans rated about 100CFM, and a 140mm pusher fan on the cooler. I think I might be able to remove at least one intake fan, leaving three. Between the CF barrel fan and the AP30 or Noctua iPPC 3000, I estimate that my exhaust is not only close to double the areal-aperture of a single rear exhaust, but it channels air over motherboard components through a narrow aperture.
The system is likely more quiet than AiO or even custom-water systems. I'm not exactly promoting my opinions or ideas, but just describing them. But I'm really really really happy with it -- it's marvelous -- and I won't release my tax returns. (I'm joking about "hyper-promotion" with that.)