The disadvantage of a longer rod is the longer the rod's length the less accurate casts become. A shorter rod will be more accurate.

The advantage of longer rods are their ability to allow more controlled mending (less line on the water) and being able to produce a higher back-cast which in turn can help increase casting distances.

This becomes apparent when back-cast obstruction becomes a problem, or when fishing from a float-tube or pontoon boat or wading deep.

The lower you are positioned in the water, the lower the casters center of gravity becomes. This tends to create a tendancy to open the back-cast up too much.

Rob