Deflect is fine.
Its hitbox is a big cube around Genji, that protects from all sides except rear.
because it’s a cube it you shoot from angles, and he is not facing you directly the shots will get deflected but away at off angles. so at least you wont hit yourself.
Melee while he can deflect. It actually just negates Genji from damage nothing else happens no reflection. so some character like brig and Doom can go to town on a Genji. with only Brigs rocket flail being affected, but Whip shot and shield works fine. and Dooms shooting being the only thing Deflect is useful against. and of course beam weapons and other stuff already mentioned.
furthermore just learn to look for and bait it out of a Genji and strike when on CD.