Hobbyist don't usually need to scale beyond one device. Warranty is not that a big issue, if you buy a used hardware you know what are getting into, that's why they are cheaper than new devices.
The author of the article mentions a hundred devices in the paragraph I quoted. Presumably they do have a goal where scaling up is relevant.
we are talking about embedded devices here. you dont need a second one because your web server cant serve enough requests, you need a second one because you want separate projects to have separate hardware.
Depends on the hobbyist, I think? The Pi has the advantage, due to the scaling, of having a rich ecosystem of examples and ready-to-go code.
It makes more sense with high availability though. Their current situation seems rough.