As an FO I would take control when when doing nothing would result in an incident. Feeling merely uncomfortable is not enough for me to consider taking over, at that point I would just utilise a standard assertiveness model. So far I have only got as far as using emergency language, in my case "you have to go around now" where he persisted with descending below MDA in IMC.
I have had control taken from me, at the time a difficult crosswind landing at night in drizzle and mist with extremely bright edge lights and no centre lights. I was lined up to touch down well to the right, too far, and the Capt just calmly said, "I have control", no drama.