In the right situation it works well. There are 3 possible negative factors IMO.
First once started you will need to continue stocking every year just like any other pond capacity booster ( feed, fertilizer, etc).
Second if they overwinter they can overpopulate and stunt and or take up to much carrying capacity .
Third if they are to numerous they can overeat the pond and cause some negative impact on BG size structure (bad year class). This of course assumes insufficient predation on forage species.