Play Pharah against a skilled Genji and you’ll know exactly what they’re complaining about. Genji can climb, Dash then right click for a kill. Not uncommon at all for him to ult, Dash Pharah for an easy kill, then Dash again on the enemy team below.
He’s a soft-counter at best, but he definitely is a problem for Pharah players. He can also get up to the little alcoves and hiding spots that Pharah likes to peek from behind and cause problems for the enemy team. Considering he almost never touches the ground, he’s a difficult target to hit with rockets as well.