In my experience so far, Auto-Plunder works as intended. However, it's hardly worth it...
The point of this feature is to save you the time of grinding, and reduce the risk of running into the resource you're not interested in.
But let's do some math. Let's assume for simplicity that the Search function costs a static 2000 and you want to plunder Gold.
Plunder requires 20 Stamina and 60K Gold. So that's 10 Battles, for each of which you pay 6K. And at level 50 (cause that's where my server is at right now) you're going to plunder an optimal amount of 80-85K Gold. And that's the best case scenario. Don't forget if it does find a mine that has like 1k to plunder, it will do it and waste 2 Stamina (Power Rolls).
If you do 10 battles manually, you'll spend 20K to plunder the same optimal amount (80-85K). You could argue you won't always run into Gold Mines, but you have to remember we're using a static 2K fee.
So we're talking 20-25K vs 60-65K profit.
So is saving 10 minutes of actual playtime worth the the 40K gold? I don't think so... And you have to take into account the fact that plunder takes up to 2 hours. Maybe more, I don't plan to test it at this level.
It probably works better at higher levels, but I still question the ratio of profit/time saved.