Some common misconceptions cropping up here.

First, the issue isn't people's computers or consoles, although those on the low end may compound the issue. The issue is the SERVER cannot handle the load of telling every single person out of 300, what the other 299 are doing, plus NPCs and mobs.

Second, if you cannot see the mob, your abilities do not affect it, at all. Same if you cannot see a party member, your abilities do not affect them, such as AoE heals.

And lastly, the few times I was at an Odin or Behemoth when they got the same audience current hunts do, they still had the vanishing issue even after the "fix".