At least one or two order of magnitude more systems are using unikernels or rtos rather than a full-blown os. You can write a small scheduler even for a small 8bit microcontrollers (avr).
The main purpose of the unikernels or small rtos are handling multiple running contexts (aka threads or tasks). I wrote several smaller kernels for embedded sytems, before starting threos.io .
It's much easier to handle multiple tasks.
ex. one of our project was a wireless voting terminal, one task handles the communication (RF) interface (protocol, error correction etc.), the other handles the user interface (buttons leds etc), and one task is handling the vote application program or logic. The tasks are communication over message passing. All of this running in a cortex-M3 with 8k-32kb ram.
A more common example are wireless RF modems (WIFI, GPRS, GPS, Bluetooth etc.), nearly every RF modem uses some kind of unikernel or rtos.
