I've only seen megacrits on level 26+ EEs since the change ,e.g. stormhorns and orchard. I've never seen any significant damage #s in the motu chain for example or other lower level epics. In EH I've never seen anything like this at all.
The devs are collecting data so they can presumably verify it's happening. I check the combat log and divide by ( 1 - my PRR damage reduction percentage) to figure out what the damage is before reduction when I can. Every time it's been melee damage from a crit and my fortification is high so I assume it's been completely bypassed. I've raised people at -500 so I don't think it's just happening to me. Others have also commented on it in this thread and in game. Of course people running EH and lower level EEs won't experience it. I am running almost exclusively higher level EEs and am one-shot about once in 4 quests. It's not like we can't get through the quest, I just think one-shotting is a stupid mechanic.
As far as rarity, people are going to be hit hundreds of times over a play session and with complete fort bypass that is going to mean several crits. What is causing the high damage boost is not known to me, but the combination of the complete fort bypass and damage boosts should be looked at by the devs. And yes having true seeing + megacrits means wizards/sorcs are especially vulnerable since displacement is a significant part of their defense and PRRs tend to be lower.
I've played 2 other MMOs and both have warning circles if an attack is coming that will result in death. I don't think it's an accident that they don't allow you to get one-shot at all in the most difficult content unless there is a warning.