Rubber roofs don't leak! Had one for 20 years that didn't leak, but left a hell-of-a lot of white streaks. Fiberglass roofs don't leak either! Aluminum roofs don't leak either! Had one for 7 years! Shipped to Europe and back as deck cargo. No leaks. Penetrating any of them with a cutout or screw that isn't sealed properly creates a leak.
As for fiberglass, the thin stuff on many of the less expensive RV's will probably not last as long as the newer "rubber" roofs. The fiberglass on a Rexhall, however, is almost 1/4 inch thick and has a jellcoat surface like a boat. Still had a leak in mine - from a bolt on the A/C that wasn't properly tightened, and an improperly sealed clearance light.
The most important part of an RV is the functionality of the house part for the particular user.
Slamming an RV for it's roof type shows a lack of understanding of priorities, and misleads less experienced users.