Though I agree that reaper would be one of the stronger OW characters if they were real, The person who has the power to revive multiple people from the dead at once (junkenstein's revenge) would have to be the strongest character. Mercy might not have that much firepower, But with an army she could take over the world. (also she could fly over reaper's shotguns)
He's not stealthy at all. Incorporeality on demand (ignoring CDs) is powerful for sure, But without the agility and reaction time of someone like Tracer it doesn't really do much. Aside from that, The totality of your kit being shotguns doesn't really allow you much impressive means of ingress OR egress a la Sombra or Pharah. Unless you're mowing down redshirts there isn't much regen potential either.