I don't mind 10's UI; the issues I have are more with usability stuff - the control panel and Settings app being separate, and the Control Panel being kind of hidden, the file association system not working properly if any slightly old program tries to change them itself, having to run third-party applications to turn off telemetry, allowing the user extremely limited control over the update system, to the point that it will sometimes just straight up restart the computer when it's still being actively used, and a number of other smaller issues that make it enough of a pain to work with (to the point that when I'm eventually forced to use 10 because of a hardware upgrade, I may end up using it purely for games and dual-booting some flavour of Linux for everything else).
These are the major problems I have with 10. The other two are accidentally changing the file manager settings when no OS older than 8 did so, and the fact that the OS keeps giving bad data to the sound driver. I end up having to put together sound effects on a different computer because I can't get the OS to leave the audio alone. All the hardware is less than a year old, so you can't blame its age.
The Windows 10 GUI was always vastly superior to Windows 8.1; I've used both. But that doesn't make the GUI of Windows 10 good to me. It still has quite a long way to go.
And whether or not the GUI is good, the sound issues are inexcusable.