As galio, If he misses his E, the enemy team can quite literally walk up to him (not in melee range) and keep poking and harassing him for a long time.

Can’t do much if the enemy laner wards the river(if he is mid lane)/lane brushes(if he is supporting). As soon as the enemy sees Galio moving out of the lane, they will start playing safely and negate Galio’s threat.

Galio’s effectiveness is greatly reduced if the enemy team positions and spaces correctly during a fight. He is decent against magic damage but AD ranged champions can tear through him, especially after he uses his E. As galio, If his E is missing, the enemy team can literally walk up to him (not within control) and continue to poach and harass him for a long time. Can't do much if the enemy keeps the river (if it is halfway) / track brushes (if it is in support). As soon as the enemy sees Galio coming out of the way, they will begin to play safely and deny the threat of Galio. Galio's effectiveness is greatly reduced if the enemy team positions properly and the spaces during a fight. He is decent against the magical damage but the AD champions can tear themselves through him, especially after using his E.