I will say that the end game crisis was really exciting in that respect, at least early on. Somehow, I'd managed to avoid war entirely for over 200 years, since I only had one neighbor that didn't like me, and I was able to keep enough fleet power that I guess they didn't think they could win.
Then a sterilization hub appeared in one of my systems in 2455, and the fleet and troop transports obliterated the outpost and dropped like 20 synthetic armies on my planet and habitat. It was actually kind of depressing watching the little army health indicators dwindle down, then see the red skull appear on all of my pops on those planets. Guess that's what I get for modding in a cute species to play as.
First attempts to retake the system were wildly unsuccessful. I had about 50K fleet power in total, and my four fleets were wrecked by the one autonomous cluster hanging around. I had 50K minerals stockpiled and plenty of spaceports, so I rebuilt quickly, and with the help of my federation ally I managed to destroy the cluster and retake the system with heavy losses. After that, I learned to use the ship designer to make ships to counter the Contingency, and had to change my entire thinking to focus on warfare instead of keeping my population happy. I ran huge energy deficits in my core sector for that entire century, and only survived because of trade agreements and by draining other sector stockpiles periodically. I had to use the Droning Optimizations edict for the first time to keep a positive mineral balance, and influence was even more scarce than usual, especially since I wanted to take systems that the Contingency had opened up in their wake.
I was actually concerned that I might not win at the last sterilization hub. I guess they get a bonus rate to fleet spawns as you destroy hubs, and they were cranking out fleets fast enough that I could barely maintain a foothold nearby. I spent several game years with four fleets parked near a wormhole so I could build up a big starport to crank out 4 battleships at a time, but sheer attrition destroyed the fleets and that starport... which I had to rebuild. Thankfully, by that time I'd destroyed all of the wandering fleets that had built up in that quarter of the galaxy, and established a foothold that let me win.
I can imagine that the Contingency won't be as fun to play against next time, so hopefully I'll get a different crisis. I wonder how they compare in difficulty, since from what I gather the Contingency is a bit more gimmicky in that they only use energy weapons and are a bit easier to hard counter with shields.