Nah, it was a great event, I Was there on the first server that did it
but they were hardly unkillable, mostly the server was lagged to shit and literally never recovered. Medivh broke so thoroughly during that event, that we went from a relatively lagless, though very high pop server, to literally unplayable at peak hours. Was pretty sad really, two of the top guilds in the world were there and had to transfer away when we got free transfer offers during Naxx. Like, if anyone pulled Thaddius it literally crashed everyone on Medivh in Naxxramas.
Similar anecdote that I doubt many others remember, but right after the first honor patch came out that had rewards, there was a banner you could use that gave people some stamina or something. At the time everyone was still working on Ragnaros in MC, and the servers kept crashing when people were pulling Rag. Eventually we worked out with the 4 other guilds attempting it A) to give everyone a heads up before every pull, and B) after many hours/days of this, that it was the fucking pvp banners being used that were crashing it. Every top guild had someone with it that was using it when they could, and every time it got used it would instantly shut down all Molten Core instances.
We also came back up from one of those crashes in the same instance as a horde guild (I was alliance), which was entertaining, and finally in BWL a few months later when nobody could get past the busted gate after Vael, the server crashed and we zoned into the same instance as Ascent, another horde guild. Good times.