> Separation of concerns is great, but if you really want different parts of your system to have different reliability constraints, then you need to truly separate them, for example by running them in different processes. Running them in the same process but having some of the code be more lax about state because it's less important is just begging for horrible bugs.

Indeed, and you might even want to run them on entirely different and independent systems (connected by some type of network/bus) depending on exactly what bits are critical vs. non-critical.