You don't need a microcontroller to build a realtime system, it could work equally well on an ARM or RISC-V machine. Determinism is key, not feature set; however, you will want to avoid modern features such as dynamic frequency scaling or the x86 system management mode that operate outside of the control of the operating system.
fair, I left out what I really wanted to say. The simplicity of a microcontroller means it's hard to shoot yourself in the foot when building realtime systems.