I bought a used Crest 24 foot pontoon boat in Texas, knowing it had some barnacles to be removed. I got them off with a pressure washer and used some Toon Brite or whatever to clean it. Seller assured me there were no leaks in the logs. Har Har. There were so many leaks that the logs were junk. It was a nice rig with a 4.3 Mercruiser and I felt lucky it didn't sink when I left it overnight.
Those typically rot from the inside out. Once water gets inside a pontoon and sits for extended periods they will rot.
It's all a care and maintenance issue.
Glass boats seem to dominate the South East.
I think historically they were the easiest to repair and maintain when aluminum welding was a rare event.
As more welding shops started working with aluminum, aluminum boats have become more popular.
I have boats in Florida, Montana and Alaska and aluminum is my first choice.
In the parts of the country that experience long frozen winters, a glass boat left sitting outside can fill with water, freeze and split wide open....