I've grown up with both shape note singing and regular "round note" singing. Shape note is best for music that doesn't change key, and doesn't use many accidentals. The more "modern" the harmony, the less useful the shapes are.

This is because a shape note singer learns to associate the shapes with the degrees of the scale. If a good shape note singer is given sheet music with just the shapes, with no staff, they can usually still sing it. But a shape note singer must "switch gears" every time the key changes or a note outside of the scale is used.