The PIO is a very neat feature, allowing (limited) hard real time programming (even when using MicroPython) and while perhaps uncommon, it's not exactly unique: TI Sitara MCU (as used e,g, in Beagle Bone Black etc.) offers more capable Programmable Realtime Units (limited functionality co-processors). The Raspberry Pico board is however much cheaper (not sure about the price of the MCUs).