To B or not to B

There is one simple thing that makes "B"s quite attractive. I read about catastrophic damage due to water leaks left and right with "C"s, "A"a, and towables. With "B"s, usually the worst I read is some stained fabric. I have never read a horror story about mushrooms growing in a "B"'s carpet, but I have read about this with other types of RVs. "B"s seem to have far better protection against water intrusion than virtually anything else out there. Especially with the fact that the roof is NTSHA-approved steel, while most rigs, it might be a thin piece of fiberglass... or even a garbage-bag thin layer of rubber that is supposed protects your expensive stuff from getting wet and full of mold growth... but usually fails in a few years (small twig puncturing it, caulk drying up, wind driving water through the caulk, etc.)