I think people are pretty grossly exaggerating how much of a nerf this is. It’s literally one interaction that can only come from two different hero’s ultimate abilities. People are acting like Wraith Form does nothing now but give you a free reload or something – remember that even in a Flux or Graviton, Reaper is still immune to damage for the entire duration.
And the logic behind it is simple, which is that the “Fade” property is getting a change overall. The only aspect of it being changed is just that they can’t escape these specific root effects, which is honestly more consistent than live, because the root effect of Junkrat’s trap, for example, will pin down faded targets and prevent them from moving.
I really don’t think this will be a huge deal for Reaper, at least not relative to his bigger concerns? It’s not like Reaper was too often escaping these ultimates and then doing something while his team was stuck, he could definitely get back into a battle or reposition, but more often than not, he was watching his team die without him. It is a nerf, but not really a significant one – you literally can’t notice this nerf if the enemy doesn’t happen to be rolling Zarya or Sigma.