I think the latter is the cause of all the graphic problems I've been having. If I use the fglrx graphic driver (for AMD/ATI), I cannot sleep anymore (it wakes up to a black screen) and I don't have ctrl-alt-F consoles anymore. If I use the xserver-xorg-video-ati driver, I cannot unlock the screen (it loops back to sddm). Which makes having a laptop rather useless.
And there are plenty of other issues: opened windows are lost between logins (or moved to random places, and always to the 1st desktop), all opened konsoles are lost, kate doesn't reopen files, some login screens are all white. Or all black. The date on the clock is too big and doesn't fit ! And one thing that ails me is that your preferences are not kept between KDE4 and 5. You have to spend an hour or way more to go through all the options to try and get the desktop the way you want it again.