It has been my observations that the efficacy is directly correlated to the amount of emotional investment an individual has, the degree of hardship the DFing will have on the individual, and how much they're isolated from general society.
The ones who were DF'd that came back usually had a majority of family who were JW, or lost a job (they were working for a JW), and didn't have an outside ("worldly" friends/family) support system.
It's emotional and psychological blackmail.
Thank you Lady Zombie for your post. It seems on the money to me.