When shooting this wide, you really need to check the whole frame in the viewfinder, and then check it again before pressing the button, its so easy to get something in the frame that you didn't intend. Its also way to easy to get too much sky or too much foreground, rendering the image less interesting, I think as a landscape lens it's quite hard to use, but you can certainly get some dramatic shots especially if you change the viewers eyepoint, to something low down, or high up. In an urban environment its probably easy yo use, as the buildings/architecture usually introduce a decent element of height.
I briefly owned the 12mm, but prefer my XF14mm (the 'classic 21mm) FOV, but I find that often now, I'm shooting landscapes with the 23, 56 or 90!!! Though the later brings it's own challenges regarding DOF.