Earlier quoted context omitted.
Writing real time serial DAQ applications that will run for 20-30 years on end on custom 20 year old microprocessor isn't exactly for the faint of heart. Source: this is part of my job, I work in fuel systems.
I meant more in the sense that it would make all the subsystems of the vehicle subject to review by security researchers and hobbyists. Things like backdoors in key/locking mechanisms or even gross incompetence would be difficult to hide when the underlying hardware and software was open source.
The issue becomes when opening sourcing engine components it becomes very easy to damage engines in ways that could injure the driver/other people on the road, as well as maintain EPA standards for emission and fuel consumption.
Not every mechanic is a Fuel System Engineer, its relatively easy to hydrolock an engine. If you screw up the internal mechanics then just a hop-skip-and-a-jump to a VERY rapid dis-assembly.
It's a lot like overclocking, but instead of a 500CPU you have a 5,000 6 chamber bomb.