I have a system setup to get its users via NIS/yp from our accounts-server.
Logging in via tty is not a problem and works perfectly.
From there, I can also run startx just fine, so the WM starts.
If I now use SDDM, a login just produces a new X11-Session with the typical "X"-Cursor.
On the other hand, logins of local users work just fine in sddm.