Both their functionings are complex, so people can get impressed. But for both of them, all the complexity is inside the device and there isn’t much to put together; and the way they hook up to your house is really simple.
Why YSK: so you don’t live too long with a broken toilet or garbage disposal, thinking it will be too hard to replace. Those two are some of the simplest things to DIY.
American here, at least in my area (upper midwest) they’re pretty rare too. I don’t have one, and I think I’ve seen one or two houses in my life that actually do