[arthurdenture]

One limitation is that the shapes are really only useful for singing, not for instrumental music. They also would not be meaningful for music that switches keys or that isn't tonal (i.e. isn't in a particular key at all). So they are a very good fit for what you might think of as "church music" (at least written before the 20th century), but for other settings they would be either meaningless or a hindrance.

That's one reason shape notation never caught on; another reason is that the notation was introduced by publishers of American sacred music and not by, say, music conservatories.