I would think they are grossly over estimating the amount of units that leak. I have camped all over up here in Alaska and I have not come across one person that has complained of their unit leaking. Typically most fiberglass constructed trailers can only leak at the corners where the EPDM / Rubber roof meets with the side walls or where they caulked around Roof Vents, AC Units, Fridge Vents etc. Most manufacturers will caulk the bajeebus out of those areas to avoid warranty issues.
As for the slide-outs my father actually had one of his beams that supports his super slide on his class C give out. They did a "repair" weld when it was new and sent it out, after two years of use the slide literally dropped about 4" and was starting to crack the exterior fiberglass. However the dealer he purchased it from took his camper in for the winter, ripped off the entire side wall of his camper and replaced all structural pieces required and built it back up. He hasn't had a single issue since then.
My best advice is make sure the repair facility you are taking it too is one you trust. Is the place you have taken too the only option you have? Northwood MFG allows you to take their units to any repair facility of your choosing, you may have the same option with your Denali.