Don't get psyched-out with cliffs, they are one of the easiest things to paint.
The cliffs in your drawing look fantastic. But you obliterated everything when you applied paint. The point of a grisaille is to let much of it show through. This is done with layers of glazes. If you completely obliterate your grisaille, then there was no point in painting it in the first place. I use the grisaille method a lot and used to have the same problem you have here until I fully understood how glazing works.
You might try painting your cliffs in a mid-tone, with slight value changes to define crevices, etc, and then, when dry, use a transparent glaze to darken the shadows. This is the old master technique of using transparent paint in dark areas and opaque in light areas. Use the darker glaze to define cracks, crevices, etc. Do this in steps and let the painting dry thoroughly before working on it again. For highlights, use opaque paint as needed. With glazing you can finely tune everything--shape, values, color/temperature, etc.
Again, your drawing looks about perfect; if you can draw cliffs that well you can paint them. I don't like giving C&C because everyone has a different opinion/method that works for them, and you can end up more confused than helped. Disregarding everything I say, just keep practicing, you'll get there soon enough.
The cliffs and rocks in the small study below were painted in the method I described, with mostly glazes. . .