Lets see.
Sound - it comes out of a random port sometimes with a sound source that dynamically appears (at a default of MAX volume in the mixer) that by the time you open the mixer app to turn it down is gone.
Multiple screens - multiple hells. So what happens when you go to sleep? Screens can swap, sometimes they change relative position, sometimes they simply become inaccessible. Multiple screens with multiple desktops? Now you're really playing with fire.
Fonts - mentioned elsewhere. You can go back and manually recompile FreeType to use the patented cleartype code (which I do, every damn time) and that code is long since clear to use.
Drivers - oh wait there is a blue tooth chip on your motherboard? Now all of a sudden the bluetooth daemon wants to mate with all your devices, whether are not you like it.
Networking? Passable if you want to use DHCP but what happens when one of your wired ports wants to be fixed address? It must be your primary port of course and gee it is configured but you can't NFS mount over it, oops, guess you didn't really want to automatically mount your NAS storage.
I can believe I'm over selling it but Kubuntu became unusable on my NUC when Plasma 5 came out, Xubuntu works ok but the screen switching is maddening. I went into the motherboard BIOS and had it disable the bluetooth in order to keep stuff from trying to bring it online and to mate with things. If I boot Windows 10 on this box (as challenged as that is) it is a better experience.
I'm really looking forward to Ubuntu on windows 10 kernel, that might have a shot at doing what I want.