Fortunately, it's a single no-frills MCU running the Zephyr RTOS. It does communicate with another system, but they are so very loosely coupled to the point that we really don't care whatever is running on the other side.

I won't get into details, but in some of the horrors stories I've heard the distributed system happened to be entirely software in nature. There are plenty of creative ways to mess up an upgrade on a uniprocessor system.