The shorthand is based on sounds, not spellings. So the difficulty is that the brain has to translate spelling into sounds first, which is a great barrier to overcome.

There is a similar fast writing system (速写）in Chinese. I tried to learn it for a year while I was in high school, but gave it up eventually. Other than the extra difficulties of logograms vs phonograms, another barrier is that one need to translate the fast writings back to "normal" ones for easy reads later. Many times it is so hard to tell what I actually scribbled down.