From what I've seen of Overwatch gameplay, there doesn't seem to be many hitscan weapons. Mostly I'm seeing various projectiles.
Yeah, and for most of the hitscan weapons, it isn't a big deal. But if you're aiming at a small head, moving from point A to point B, it can look something like this crappy image:
The yellow line is the actual path the target took, the orange line is the shot that was fired, and the green dots are the data points the shooter received from the server. The red circle is where the target's head would show up at the moment of the shot, and the magenta is actual position (the server's data).
Here the orange shot will clearly hit the head from the shooter's perspective, even though the server (with 3x as many data points) knows that the head wasn't there, so it registers as a miss. The difference is usually too minute to matter, and the intervals are definitely too tight for reaction time to play into it. But since it's still perceptible, people will miss shots their brain knows they should've gotten, and have no idea why, and hence will never be able to anticipate these sorts of jagged movements in the future.
For a large hitbox moving slowly, it's extremely unlikely to matter, but if it's a tiny head hitbox moving quickly in weird directions, 20/s interpolation can be frustrating. In a game like CS, the 120 competitive server tickrate is pretty essential. In TF2, I wish 120/s was possible. In overwatch? If the hitboxes really are that massive, they can probably get away with 20/s client interpolation, even with the sniper.