I'm not sure how inconsistent UI equates to buggy but I see this argument a lot and it's mostly unfounded. Those inconsistent UI screenshots are always different versions of the OS or different themes. I made my own comparison 2 months ago to show that it's not that bad.
The other annoying thing is that this is portrayed as a Microsoft thing. Linux Desktop UIs are horribly inconsistent but it's somehow ok. People just about lose their minds when Apple updates the UI of iTunes or Safari out of band with the rest of the OS.
Microsoft hasn't done much to bring GDI UI elements up to Modern UI specs and that's the main pain point. The consistency variations between UI elements in Modern UI apps comes down to individual Apps doing different things. That happens on all OSes with vendors like Adobe all the time.
That example is either out-dated or was not made with good care. On my system there are three types of menus and within each group they are consistent: http://i.imgur.com/ig4gvgc.png
- the ones Edge renders
- the ones the taskbar renders
- the ones rendered for pure window chrome
Also, some details between the groups differ, but they're still rendered with the same code and UX. What differs is only the styling, colors, padding, margins.
