The idea of modern UI was good. The traditional freely movable windows are usually not using the desktop space very efficiently unless you spend a lot of time positioning them or use some third party utilities and configure them properly. I have a feeling that especially "normal users" very seldom use for example two windows side-by-side - even though this might be good for productivity.
The modern UI style was more like a tiling window manager. 8.1 made it possible to split the screen vertically among n applications. It would have been nice to see them explore this further to allow other kind of configurations as well.
The main problem with modern UI from my perspective was the lack of application. There was only few modern UI apps available and most of them pretty useless, tiny utilities. Now Office (even Outlook), no Visual Studio etc. A "simple" fix would have been to allow users to run the old applications the same way as modern UI apps.