QNX is a good OS but Blackberry devices were no better from using it than Android for using Linux, iOS for using BSD, Windows Phone for using NT. All these devices managed to present modern, efficient user friendly phone experiences. And that's all that matters from a user perspective. The choice of micro or monolithic kernel is a sideshow.
As for IoT devices, I expect the main driver for what kernel / OS they use is what tools are available, how well they work, how much they cost and what if any runtime license does the manufacturer have to pay per unit. Since the tools for Linux are generally excellent and the runtime cost is zero, it's clearly going to be the defacto choice unless there is a reason to choose differently.