A weakness electrical systems and biological systems both have is they dont always know where the instructions were issues from, so if the can bus is viewed like a nervous system, injecting messages can cause problems especially if the device doesnt verify the authenticity of said message. Biological systems cant do this, but digital can take steps to improve the authenticity of said messages.

In a way its a problem CPU's suffer as well, when thinking of code that pretends its a USB mouse and keyboard and then destroys stuff off the computer whilst masquerading as the user. Until such devices have self preservation built into them as standard, the ability to hack devices remains easy.