I have used 20mm lenses for landscape for years and the f/1.8 is a great one, particularly when you take into account its price.
The lens is super sharp upto f/14. I shoot Landscapes at f/11 so perfect for me. It is also my no-one walk about lens on a stripped down D810.
The Nikon 24mm/1.8G is said to be sharper, it is, but the field of view is simply not as appealing for me.
The much larger, heavier and more expensive Sigma Art 20mm f/1.4 is wonderful, but so much more lens.