QNX is in a space with few competitors. FreeRTOS or ThreadX are designed to provide microcontrollers with scheduling and memory management functionality. They don't depend on fancy things like MMUs or provide frameworks for networking or file systems out for the box. The flipside is that you can compile them down to maybe 30kB of machine code.

QNX is designed for more powerful and featureful hardware to drive a software stack with true process isolation and generally provide the bells and whistles of general purpose OS on top of a hard realtime core. It can run complex GUIs without sacrificing its real time capabilities. Not many competitors live in that particular space.