Like most words musicians use, the meaning of cantabile has changed a great deal over time and from one side of the Alps to the other.
To modern pianists it seems to conjure up bel canto opera arias. To a Baroque-trained singer, however, it is quite the opposite. Baroque vocal style relies heavily on a sense of the center of the voice and of the vowel. Notes are not generic, pitches and range are functions of the animal organ that is our voice and we should make no effort to gloss over them.
The cantabile instruction that Bach gives his son Wilhelm Friedemann in his preface to the Inventions makes sense only in Baroque vocal terms. It is plenty difficult to discern nonetheless.
