That may be true in many cases, but I find ubuntu/gnome is way out in front of windows 10 when it comes to UX. Windows 10 is all over the place, every app looks different, some are full screen others aren't. Half the OS is touch friendly but the other half isn't. All sorts of functionality is hidden in slide out menus.
If this is the product of UX designers then I'm glad the OSS world doesn't have any.
I'm not sure this is a major issue. End users are mainly concerned that when they wake up in the morning, lots of stuff hasn't moved. What it looks like and where it is to start with doesn't seem to matter as long as the UI is discoverable to some extent.
End users are also concerned with not being frustrated at every corner with arcane and weird UI choices or a user experience that plainly mirrors the implementation choices underneath. Worrying about whether features have been moved around the UI usually comes after they've already used a program for a while. There's lots of open-source software that, for non-technical end users, doesn't get to that point because they've ditched it already.
I'd say that the OSS UX will always lag in terms of overall polish. But in terms of pushing ideas, they can be ahead of the curve. E.g. gnome 3 (April 2011) did a full screen "start menu" before windows 8 (Aug 2012)
If this is the product of UX designers then I'm glad the OSS world doesn't have any.