Shonumi, New article about the current state of Game Boy emulation is live:
https://shonumi.github.io/articles/art34.html
The article dives into how well many peripherals and accessories can be emulated. It focuses exclusively on officially licensed hardware that software interacted with. In brief:
Most stuff is emulated just fine (37/47)
A small % needs just a bit more work (9/47)
Only 1 item is completely unemulated
We've done a lot of work over the years, and it shows!