Well... My last real challenge was in 1.6, when Awakened Empires were truly powerful.
I was playing as a Fanatic Purifier arthropoid species that despised its own organic form, so we pursued and achieved our cybernetic ascension. A militarist fallen empire (they don't exist anymore, do they?) suddenly awakened and snowballed everything in their path. Empire after empire fell to their mighty fleets. I had the opportunity to eliminate them just before their awakening, but I was recovering from a successful campaign against other fallen empire (which I managed to destroy completely). In the end: GAME OVER.
Maybe the fallen empires were too powerful back then (after their awakening, and even before that), but now they're too weak. Since 1.8 - 1.9, I don't see a War in Heaven - I destroy them before a possible awakening, looking for their juicy unique techs.