Smudges and dirt on the screen can lead to errors. Also has problems with ambiguous imput. Good for novices, walkup. Minimize the consequences of inadvertent button selections. Most handwriting recognition systems has some sort of context dependency.
Some systems have adaptative training capabilites.
Useful for mobile interfaces. Complex Gestures Utilize 3D space of physical movement. Issues of resolution and response. Which are implementation dependent. There is no technology on the user.