Whoever said to fiberglass that has no idea what they were talking about. It’s a seam that the forces applied to the transom are able to release without transferring to the top cap. fiberglassing it over, doesn’t make those stresses go away. Essentially forces a static condition for a dynamic force. There is now no where to for the stresses to release causing the crack.
similar to bride joints and rollers. You don’t see the bridge move, but it does when loads are applied. if those joints and rollers don’t exist, the foundations and deck would fail because it’s now a fully static support for a dynamic load.