I've had two Stern Savers on my Champion's transom for a few years.
One is used to mount a Lowrance HDS skimmer transducer on back of transom, the other mounts a Lowrance StructureScan LSS1 transducer on the roof of the transom step.
Installation requires cleaning and scuffing the gel coat with sandpaper where the Stern Saver is mounted to allow proper bonding using the provided special 2-part epoxy.
Video if you've not already seen it:https://www.youtube.com/watch?v=ciy1najrIFE
Both Stern Savers and transducers are mounted solid with no sign of coming loose after being mounted for a few years. I have no idea what would happen to gel coat or fiberglass if I tried to remove them but I consider them to be permanently mounted. Check the YouTube video comments.
If you have a wood transom using these is far better than screw holes in the transom IMO.