This was downvoted quite a bit when I came across it, but can someone explain why it's so bad?

Sometimes you do want to add GPIO to an existing Windows machine, but if you just want to mess around with .Net and some simple electronics, the parent's suggestion is cheap and simple (and e.g. https://github.com/raspberry-sharp/raspberry-sharp-io claims to support this use case.)

Only the Pi 3 (and CM3, CM3L presumably) are supported iirc.