Generation of stepper signals or interfacing with servo amplifiers and encoders is best left to an external microcontroller of FPGA card, connected either via SPI or ethernet.
While I admire the ingenuity of people doing real-time with Linux, I just shake my head at the man hours wasted that they could be doing the "thing they want to do" instead of fighting Linux.
The PRUs are just the replacement for the FPGA resp. microcontroller. You have your relatively relaxed control loop running at e.g. 1kHz on the main ARM and the tightly constrained stuff on the PRUs, not much difference to running it on something external connected via SPI. Some kind of external hardware is needed anyways, doesn't make much difference including a STM32.
The problem with beaglebone is lacking GPU afaik.
Only macOS has an architecture to deal with low-latency audio and video properly. Presumably iOS has some of those features as well, but it was on OS X that Apple did some deep architectural changes for proper low-latency audio and video.