Opinions, eh? Not a problem, always got one. As to usefulness, who knows?
I've had underdeck aluminum, built in place fiberglass, portable fiberglass, portable metal,
rotocast underdeck and rotocast portable. Installation is everything, including the portables.
Don't want tanks shifting postion, visibility to all sides and bottom for inspection.
Protected from spray, slosh, splash, drips, leaks to prevent water contamination of fuel.
I prefer 2 tanks, not one large tank. One tank for on the way out and exploring, the other
for the shortest distance back to the ramp/marina. Also convenient for those trips,
where the fuel system has a problem, switch over to the other tank to get home.
2 sets of fuel lines, 2 fills, 2 vents, tanks not interconnected. I like redundancy.
I prefer rotocast tanks, portable or underdeck. Just have to ensure the top of the tank stays dry
and that all hoses have the required loop heights and vent/surge stops.