[admford]

Oil cooling in general is more useful in overclocking situations where one uses a TEC. The main problem with TEC cooling is that you can get under 0 degrees, and that leads to condensation on the logic board (not something one would want). Mineral Oil freezes at around -30 degrees C, so you can cool components below zero without having those problems. Though if you're using TECs for cooling, then efficiency isn't something that one would be interested in.

Otherwise, mineral oil needs pumps, and in general could be harder to design cooling components due to the different density of the fluid. High surface area coolers that are used with fans might not let the oil pass through fast enough to limit heat accumulation. You'd need to design laminar flow heat sinks and pumps that keep the oil flowing over the most critical parts. If one pump fails, you risk localized overheating. At least with standard fan/air cooling you can have multiple fans that keep the hot air from staying in the case (or some racks have A/C vents entering from the bottom front and exit from the top rear to keep positive pressure and continuous cooling.

The limiting factor could be logic board design to make sure there's no turbulence between the components and cooling equipment so there aren't any hot spots that could damage components. It's costly engineering and you risk starting to make or use parts in non standard sizes or designs.