There are a lot of ST riders who have put one of these on their ST fenders. I also have known many GW riders with something similar. My own GW came from the PO with a mud flap of his own design made of rubber that was bolted on. I don't recall reading of any issues with the stock fender with these. My own experience is only with a stiff rubber mud flap. Mine started tearing, the mud flap not the fender, and was beginning to rub on the tire so I removed it before any damage was done. No splits at the bolts though. I also had a rubber mud flap on the rear fender that originally had a small light in it. The wires were broken so I removed the light but left the flap. Never had a problem with it otherwise. I do know of many add-ons bolted to other plastic on a GW that had splits starting around the bolt holes but usually related to too much weight or stress applied, such as on a luggage rack bolted to the top case lid. I added large fender washers inside and outside to take care of that on mine. So I can see where there could be issues. But usually it's related to the bolt hole with bolt-on items rather than stick on items such as a Fenda Extenda. Just another view here.
BTW- tried looking up for this item on the official Fenda Extenda web site and it is not listed... at least for the U.S.A. Be wary of eBay claims of compatibility. I've know items advertised on eBay for one model bike are really intended for a different model and don't fit. But then again, maybe it
is available overseas and just not here.