Well, as you say, desktop environments look the same. But they don't feel the same. When we talk about UI/UX we talk about look and feel. I've installed Ubuntu & MX Linux to reply to your question, here are random issues I get with the default setup: why do I get no sound when I play a youtube video (it works on one distro, broken on the other one) ? why do I have no option to change the refresh rate of my monitor to more than 60Hz ? Therefore where is the 120hz smooth scrolling ? Why is my keyboard layout set to english by default while I am located in Europe ? How am I supposed to configure my WIFI password if my keyboard layout is incorrect by default ? Why does my CPU fan runs at full speed while I am just writing this text currently with no other user task in background ? If I use a laptop with a second keyboard (bluetooth), why aren't my keystrokes taken into account when the system asks me to confirm the reboot by pressing "enter" (I need to reach the laptop keyboard since the bluetooth one is not taken into account) ? Why does my browser need to reload its cache & configuration from my drive each time I boot my computer; why isn't it loaded in RAM by default ? Why do I still have the first mouse wheel scrolling event ignored in the Chrome/Chromium web browsers in most distros I try, while this bug has been driving users nuts for years while the solution is already known ? I use a dual monitor setup, why does the system displays some glitches at first boot ? Why doesn't it doesn't remember my primary screen when I log off the session, and shows the login prompt on the other screen ? Why does the system hangs when I run a game like counter strike 2 and I try to switch back to the desktop ? Why does the system get super slow while it tries to recompile shaders, making it impossible to open a web browser ? and I'm not even mentioning the issues installing/uninstalling proprietary video drivers which is still a huge mess. For my usage, the Linux Desktop feel experience is still pure hell currently... although I really wish it were good.
