I have tried the laptop/PC-remote combo and it's bad. Plex has an HTPC build which is okay, but restrictive. She also uses YouTube and other apps, and we find the Nvidia Shield has the best UI and performance. Computer OSs also have a bad habit of stealing focus, updating, and overlaying notifications. These require a lot of effort to disable, and in Windows, it's impossible to get it all working like a dedicated streaming box.
I take your point above, though. Plex is trickier than streaming. So if the use case is laptop or maybe tablet, those streaming sites are great.