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The PIOs are much smaller and less powerful/flexible than the PRU. The PRUs are 200 mhz 32-bit cpus with DSP-like capabilities and 8k of memory (partly shared with the application cpu). They can directly access the analog ports too. I don't know if the PIOs can do that.
Each PIO is limited to 32 instruction words (16 bit words; the 5-bit address is baked into the instruction format so it can't be expanded without redesigning the ISA), its memory consists of some 4-word (32 bit word) FIFOs and two 32-bit scratch registers, and the instruction set is very limited. It can increment so you can do counted loops, but it has no ADD instruction, so e.g. it can't easily compute a packet checksum on its own, much less anything like an error correcting code. It is nice for basic bit banging of things like the Neopixel LED. Someone did DVI video with it (https://github.com/Wren6991/picodvi) but that was really stretching its capabilities, seemingly more of a flex than something really practical. Impressive though.
It's also odd that they used the M0+ core instead of an M3 or M4F, especially since they then bolted on an external divider/interpolator to speed up audio processing. Maybe future versions will upgrade. Perhaps even to risc-v ;).
I've been wondering whether the RPi foundation designed the PIO from scratch for this chip, or if it's an existing block they got from someplace. It strikes me as clunky and maybe old, coming from an era when transistors cost a lot more. As for the chip itself, it's always nice to have a new cheap MCU board, but it's hard to tell what they have in mind for it. A traditional single-core MCU would have been simpler and probably cheaper, and something aiming for more compute power would have been better off with more powerful cores (future versions might have that, and more cores too).
The coming ESP32c3 will supposedly be priced like the ESP8266 (i.e. $2 or so for a module) and it will have a RISC-V core and 300K of ram or something like that, plus wifi, so it in some ways seems more promising. For boards, I like the Longan Nano on seeedstudio, like a RISC-V based bluepill for $6 including a tiny TFT display. Unfortunately it has just 128KB of on-chip flash so it can't run micropython simply, but it has a microsd slot so maybe there is a way to use that.
I used a Wemos D1 ESP8266 in this project [0]. Plenty good for the project at hand. I heard they have an ESP32 version now?
For one-off personal projects I'd probably still reach for a TinyPICO or FeatherS2 though because of the additional features they have, and that they have a Discord server you can ask questions to, but if you're deploying a dozen of them in your garden and want to save money then yeah, the Wemos would probably be a good bet. Support and documentation is a little sparse though, in comparison.
PIO is definetly interesting stuff.
This has its own assembly language (.pio) to program. Custom peripherals can be implemented by using this assembly.
Raspberry Pi already provides example programs such as custom I2C, SPI, UART (they are different from the hardware peripherals in the chip), logic analyser, LED controller, and etc[1].
[1]: https://github.com/raspberrypi/pico-examples/tree/master/pio