I've seen barbarians sending out actual stacks now. They can't support proper stacks of doom, but decent sized hordes, say 10-15 units, against both me and the AI civs. They're great fun, but I wish they'd all come for me.
When I get to the exploration era of caravels and such, I often find islands or continents covered in barbarian cities, but because I have the "Assimilation" option on I can see that these cities used to belong to civilizations. I've seen it happen directly if I can get a mountaineering unit to safety near a 3+ barbarian city. The city becomes a civ with a half dozen units, and before it can build any culture defense or more units or do that thing where it develops and pops a golden age and gets a bunch of free units, a barbarian stack comes and retakes it.
Stacks containing neanderthals can sometimes be annoying too, but for a different reason. They move to an enemy city, but their -city attack tells their AI they don't stand a chance attacking the city, so they sit there, while the neanderthals high strength tells the civ's AI that it doesn't stand a chance trying to expel them, so they sit inside the city. All their improvements get pillaged by the barbarians, who occupy the high-yield plots, and they can't even send out units to hunt animals. It ends up with a crippled civilization that falls way behind.