My 2001 208 has an original 82G poly tank. Does it leak? I don't think so
I think Grady was pretty sure that the poly would outlive the hull and to prove it, they didn't put a removable deck lid over it! Yup, if I wanted to check or pull the tank, I have to cut the deck
I suppose there are advantages and disadvantages for one material over the other. The ease/difficulty of manufacturing larger poly tanks may be an issue. I really don't know.
I do know that poly tanks can flex and bow under the load of fuel. If you look up the recommendations for selecting WEMA style senders, the advice is to select a sender that is about a half inch less than the tank depth for metal tanks and a full inch shorter for poly tanks.
I would think that expansion and contraction of poly due to temp swings and gas load may be greater as the capacity gets larger and I guess that might lead to potential stress faults in plastic tanks.
I have seem one case of a poly tank developing a leak but that was on a pontoon with an exposed poly tank. I don't know i=f that leak was a result of a mechanical failure or perhaps degradation of the plastic due to UV exposure. The leak was on the top of the tank. Since the top of the tank was a bit concave, the tank didn't leak gas per se. Rather it allowed rain water to pool, seep into the tank, and displace the gas. It is more common to have issues with the sender flange screw holes. You have to be careful when removing the sender. Sometimes, the threads come out with the screw.