I've only ever lost a fort to FPS issues. In fact, I've never even had a fort be decimated by anything.
But once, something came close. Here's the story.
So, I had a prospering young fort built on a tropical shrubland. There were large herds of elephants around, so I had plenty of food. There were stacks of elephant meat piling up in my food stockpile faster than I could make barrels for them. I had several hunters that had become legendary weapon users through their elephant hunts. Up until now, my only fatalities had been peasant elephant hunters who had been killed by wandering big cats (as much as I'd like to say "I'm so badass that elephants are afraid of my dwarves, elephants are afraid of everything in this version).
That's when the mandrills came.
A cloud of vicious, screaming m's came hurtling into my base, grabbing trinkets and food. And dwarves. They would lunge at my dwarves, biting and clawing them and dogpiling them down. I even lost a few hunters to the mandrills. They just jumped on my dwarves and ripped them apart. Ambushes and sieges were nothing compared to this. Eventually, I recruited my entire population and stationed them in the midst of the mandrills.
I lost a pretty big chunk of dwarves. Probably between 5 and 15.
I couldn't build coffins fast enough for the dead, so I almost had a tantrum spiral. I barely managed to pull out of it with my delicious elephant meat dishes.
Seriously, mandrills are worse than any siege. They're like naked, rabid goblins with sharp teeth and claws.