Plus all the extra drag through viscosity on the pump etc. Some watches are oil filled but over the years they found they had to use thinner and thinner oils as they caused drag on the hands and movement.
Mineral was and still is used to totally submerge an entire system which is/was fully operational see https://www.pugetsystems.com/submerged.php
but I don't recall it being all that wonderful especially after heat transfer occurs.
It was never intended as a loop coolant fulid.