Handwriting lessons which involve drilling (such as writing squiggly lines, writing out the entire alphabet from Aa to Zz, or writing each letter over and over) are less effective.
Thin writing utensils like pencils are harder to grasp for beginners, and paper on the table does not allow for large strokes. The child may have a hard time keeping the paper from shifting all over the place.
Letter writing drills do not involve writing real words which makes the task quite boring to the child.
The above factors can make teaching letter writing a frustrating experience for both the adult and child.