QNX works on real-time microcontrollers, where you just don't have the kind of hardware that can subvert an entire system by doing rouge DMA transfers. General-purpose systems are very different.

QNX works in whatever CPU one puts it on, plenty of choice, including plain Intel ones.

QNX is for "big embedded", not microcontrollers - it's plausible to have as deployment a POWER9 cpu. So is VxWorks.