Yes, it looks like its very unpredictable. Sometimes works, sometimes not.
I used to use bane, which usually kills 1-2 people from a camp. It looks like the NPC eat from pot very ocassionally.
Sometimes only 1, the other 3-4 are passing the meal.
Even if the Cumans says in Hungarian the “I’m gonna get drunk, what else should I do”. No-one ever drinked from my poisoned wine yet…
So my conclusion is that you could poison 1 or 2 from them and have to kill the rest in regular way. Also prepare for a long time wait if you would like to use poison.
The only one which do wonders for me, is dollmaking poison on weapons. Slows down the enemy immediatelly, and prevents them from running away.