I used to use many of them on my overflows, and used to really like them since they fill up the empty spots. But once the small frags grew up, the mag plug cannot hold the weight at all, and they fell off left and right. Even for the massively ecrusted ones, since they encrust on a very smooth surface, the encrusting came off very easily and does not help holding onto the wall. I and down to last one on such a plug now, and do not plan to use them anymore in the future.
Try to find a picture with a good sized colony on it, and I think you will have a hard time.