The downside to shooting a longer arrow, IMO, is the greater likelihood that it will get in the way while hunting. In bow quivers, they stick out further, often below the lower limb of the bow, often too far to make it possible to lean/prop the bow up against a tree or other rest. A minor inconvenience, to be sure. They're more likely to catch/hit brush when the bow is held in shooting position/shot. And in tight quarters, say having to move from one side of a tree to the other to get a shoot an an approaching deer, the extra length of the arrow on the bow can make it difficult, if not impossible, to move from one side of the tree to the other. At the least, it creates more movement.