One point I recall being talked about was the fact that in order to make the patch strong enough the repair technique is such that it adds weight to the boat. Not a big deal in the case of cruisers like GBs--- the heavier they are the better they ride
-- but it was a drawback to this particular process in the view of the folks discussing it, as I recall.
This is not my area of knowledge so I can only repeat what was discussed. But to my way of thinking the big advantage of a fiberglass boat over wood is that the hull is one integral piece. No individual components to loosen over time, need refastening, have seams that can open up, and so forth. Sure, a holed hull can be repaired and obviously repaired for the long term as Capt. Bill points out. But the advantage of that one-piece hull has been lost to a degree.
We chose to "roll" the 787's composite fuselage sections to make each section a one-piece, integral piece, as opposed to Airbus' decision to make their A350 fuselage out of multiple composite panels attached to a frame. Our fuselage shells don't have or need a frame. Where a plane like the 777 has literally thousands of parts that make up each fuselage section shell, each fuselage shell in the 787;s fuselage is a single part number because it is just a single piece. Even the stringers are integral to the shell; they are not separate components.
This has all sorts of advantages until you put a hole in it. It can repaired, of course. We spent a lot of time and money developing repair processes that don't alter the safety of the structure. But the repairs add weight, much more of a big deal in an airplane than in a boat.
And as soon as you make a hole and then patch it you open the door to risk. Is the job done right? Did the materials cure up properly? Was the patch itself designed effectively? There are, of course, ways of testing the integrity of a composite structure (fiberglass is a composite). With our airplanes the primary means of non-destructive composite testing is ultrasonic. But the fact remains that where you had a nice, one-piece, uncompromised hull (or fuselage) now you don't.
So the view of the GB experts seemed to be why open the door to risk when there are methods of removing old tanks and installing new ones that don't disturb the integrity of the hull at all? At least with GBs. The discussions did not branch out into changing out tanks in other makes and models.
Their view makes sense to me, but as I say, it's not an area I have any experience in whatsoever.