To me the benefits of an MCU have to do with latency on things like interrupts. A real OS sometimes gets in the way, if you're trying to run things on very tight timing, or want to go super low power. That's why even though I'm drowning in under-used Pis, I'm using Picos to drive the lights I'm making. (Trying to coordinate multiple 3w RGB LED floods with < 10ms of latency for fancy lighting effects - because as a maker - I can do it for as little as 10 times the cost of buying it). Also, I would rather release the magic blue smoke out of a $5 Pico than a $40+ RPi. Although the Zeros were nice. We should have another round of zeros.