XFCE certainly deserves to be counted as the 3rd variant.
Since 2004, I have been using only Linux on all my desktops and laptops.
In my opinion, KDE 3.5 was a much better desktop environment than any earlier or later Linux DE and also much better than any MS Windows version.
I am excluding from this comparison tiled WM's, as they belong to a different category, not directly comparable.
The reason is that KDE 3.5 was extremely customizable. I did not like KDE 3.5 in its default configuration, but I could change absolutely every behavior detail to match my personal preferences.
Unfortunately KDE 3.5 was followed by KDE 4.0, which retained only the name from KDE and it removed all the features that I liked in KDE 3.5.
I stayed with KDE 3.5 one or two years, but eventually I had to give up because it was far too difficult to update other parts of the system without breaking the old KDE.
The rescue came from XFCE, which does not include many features, but it covers all the basic needs and it is customizable enough to be able to make it never stand in my way.
In more than a decade of using XFCE I have never encountered any problem with any GUI application, regardless if it was originally intended for Gnome or KDE.
I have tried occasionally, from time to time, various KDE or Gnome versions, but I have never liked any of them, in comparison with XFCE.
I also work professionally with MS Windows, but I have never seen any advantage there. For example, the modern Windows Explorer has a user interface far worse than any file manager that I have been using on Linux and launching an application from the Windows Start menu is far slower and more inconvenient that the kind of menus I am using in XFCE.
With Desktop Linux (or BSD), I feel you either
a) Settle and go down the route of a tile-manager (i3/xmonad/...N). It's definitely worth it !
OR
b) Use KDE/GNOME while silently swearing and hoping the next release will be better.
* As an outside option there is also XFCE (old reliable)
Always glad to see new movement and innovation in the DE space. This(HelloSystem) looks very good ! Well done to everyone involved.