Well, I mean on those vendor NASes it's pretty much just chuck in a hard drive or two, follow prompts for setup, install the Plex app from said vendor's app marketplace, make a login, and add your content to the specific TV, Music, or Movie folders...Admittedly this doesn't get you setup with running Plex outside the network, but as a basic setup it's fairly easy.

I'm also confident someone would mess those instructions up, but if you even understand what self hosting is I'm fairy confident someone could follow the above instructions to add their content. Obtaining content is a different story, but if you already have your content it is easy.