Though I am not 100% what is a mega event and what is not, I will post my thoughts.
Pirates - they come in now with lower logistics and lower weapons. I liked them. They screwed with everyone. Actually, they screwed with the Dominion of Korx more than me. My fleet was stronger than theirs so they left me alone, but where they saw smaller weaker ships (Korx at the time) they attacked them and KEPT attacking them.
Peacekeepers - actually went by their namesake: they keep the peace somewhat. Like the pirates, they had lower logistics and lower weapons, BUT they were scattered all over the place. There were as many planets as there were P.K. ships. This last part may have been a bug, but they were in minor fleets and had the "big ship" icons. Then I saved and went to bed. I woke up and loaded the saved game and every large ship P.K. turned into the smaller icon (like survey ships) and then in one turn they all dissapeared. I don't know if this was to happen like that.
Assassinations - I kinda don't like this one. In this game the Altarians have gone to war with me for this twice. The first time they ended things. This 2nd time is at a time when I am already at war with the Korx. Thing is, the Altarians were my allies!!! But the really bad thing is that they are supplied (as it says) by an unseen group. This unseen group gave them tiny weak ship but they are all over the place!!! I am going to say that this mega event hurts the immersion, mainly because it ignores current relations with the Altarians, whom I was also trading with.
Anyone else have opinions?