I've taken apart a couple of Ford automotive engine control modules and the circuit board and all the components are invariably covered with some sort of coating that feels like silicone rubber. I suspect this is what is really needed to provide good corrosion protection to the board.
Interestingly, these ECMs are installed in the passenger compartment, not the engine compartment.
The other use for the conformal coating is to hold components in place as a form of protection from vibration.
Funny story about ECMs in passenger compartments: Years ago the VK Holden Commodores (released in 1984-5) had the ECMs mounted in the passengers footwell behind the kick panel. These were the first model to have an ECM fitted.The prevoius model did not have an ECM fitted so this was the location that the fire extinguishers were fitted.
On the changeover from the VH to VK models, the fire extinguihsers were fitted onto the passengers side kick panel.
All was going quite well until the time came to start the cars to move them out of the shed. Can anyone guess what happened?