If you step up to the rim of the Grand Canyon your eye sees a grand scene. An unbelievable scene.
Shoot it at 16mm FX without carefully thinking about the forground, and you can easily end up with a very mundane scene that your eyes saw very differently. All the grand detail is so tiny (because it is very distant) that it is very difficult to make a coherent and attention riveting scene.
If you could find a very interesting rock with unusual erosion lines, in just the right light, at the rim of the canyon, and get very close to that foreground subjectthen you can start to make a nice image out of it.
That "rule" can be broken, and when I see it done with the Grand Canyon I always study the image to try to figure out how it was done. I use the Grand Canyon because it is the stereotypical example of the ultra-wide angle problem, and oft quoted. But it is true with most or all subjects.
This has nothing to do with sharpness or distortion, or other "lens quality" issues. It is purely a matter of perspective.
Some people find ultra-wide shooting relatively easy and adjust rapidly. Others have to think hard about it and many struggle for years with it.