I thought I was the only one who thought this. I keep Windows 8.1 on a Surface 3 just to have fun. Really don't know why people hated it so much. It was very usable on a desktop after the feature update. Much better on tablets than Windows 10's crummy "tablet mode." That's one awful start screen!
I don't think people or reviewers complained about Windows 8 / 8.1 for tablets / surfaces. In-fact, almost every review praised it on it's improved power efficiency, updated aesthetic & interface finishing, and lower system-resource usage over Windows 7. They even praised it for the improved user experience of the interface on tablets and surface devices. BUT most users weren't using touchscreen devices, and the user interface was objectively terrible and felt dumbed-down and slow for mouse input. I remember even after the 8.1 update on school computers, just thinking "why?" for so many dumb little interface decisions Microsoft made for everyday, non-touchscreen PCs.
On the other side of the coin, my brother purchased an RT directly from Microsoft during their fire sale of those devices, and I know exactly what you mean. The UI was, and still is in a lot of ways, the best user interface for touch screen devices. It was really, really well done in that regard. While I consider my iPad Pro my favorite device, I can't help but feel Apple is behind in terms of features when it comes to iOS on tablets, and Windows RT felt more "Pro" with stronger customization & feature set. But the devices themselves with obvious lack of developer support aren't anything more than an enjoyable relic of the past.